It’s that time of year when TV shows finish forever. Succession’s nasty media scions ended their backstabbing and bickering. Midge concluded her journey in search of comedic stardom on The Marvelous Mrs. Maisel. And the May 30th episode of soccer dramedy Ted Lasso was probably the last. Maybe you watched these finales and found the resolutions satisfying. Even so, if you’re a superfan, perhaps you also experienced a bit of despair. It’s not unhappiness with the ending of a narrative, necessarily, but unhappiness that the narrative was ending. 

If you’ve felt deflated once a favorite show has wrapped up, you’re not alone. There’s even an unofficial term for it: post-series depression, or PSD. 

“It’s a feeling of emptiness and upset when a series or something that you really love is finishing or ending,” says Rita Kottasz, an associate professor of marketing at Kingston University, London, who has been at the forefront of post-series depression research. Whether it’s TV, a book, or a video game, there is a yearning, she says, “that you want more of it.”

The difference between PSD and depression

The concept of PSD gained traction on social media and in fan blogs in the mid-2010s. “It makes sense as a non-clinical way to describe a contemporary psychological phenomenon, which we’ve probably seen more during the Golden Age of TV,” says Chicago-based psychologist Brian Kong, citing Game of Thrones as a show with huge cultural influence.

Kottasz doesn’t particularly like the name PSD, and makes a distinction between clinical depression and the more colloquial sense of being down. In a draft of her 2020 paper on the phenomenon, she called it “consumer saudade,” using a Portuguese word that lacks a direct English translation. It is a sensation sort of like nostalgic longing. (The 17th-century writer Manuel de Melo called saudade “a pleasure you suffer, an ailment you enjoy.”) Ultimately, a journal editor persuaded her to swap out the phrase, and Kottasz chose PSD because it was established outside of research. 

In the 2019 study, Kottasz and her colleagues published a 15-item classification scale for PSD, based on interviews with fans who reported sadness after their favorite things ended. She collected the most frequent emotions associated with PSD from the replies: among them, feeling frustrated, disappointed, indignant, sad, or empty inside. Some said they felt “that life is less complete now that the series is over” or that they had lost a few of their “best friends.”

[Related: From the archives: When the US first caught TV fever]

Although post-series depression suggests a focus on TV shows (a 2020 survey indicated male fans of Breaking Bad seem to be particularly susceptible to PSD), Kottasz is probing the connection to other kinds of media. Her ongoing research includes the abruptly announced hiatus of K-pop band BTS, which may have crushed young fans. It’s also applicable to novels. Millennials who grew up with Harry Potter—reading the books as children, then watching the movies as teens or adults—have expressed it. She found that “younger people are definitely more affected” than older ones, which can be attributed in part to the shift to on-demand streaming of shows and films. Business models that constantly push new content, such as Netflix recommendations that invite viewers to watch similar shows as soon as a series is finished, might contribute to this, too. “Companies are incredibly good at playing on the emotions of consumers,” she says.

Contrary to what you might expect, though, the sensation doesn’t seem to be triggered by binge-watching, Kottasz says. Instead, long-term consumption may be a factor. Kottasz thinks watching a show over several seasons or reading novels across many years strengthens a person’s relationship to the characters. In her 2019 paper, she cites a Harry Potter devotee who started reading the series at age 9 and was “cruelly left behind” after the final book and film released years later.

But it isn’t quite as simple as saying the end of a show or novel controls our emotional state. Kong is concerned that the phrase PSD might imply a causal relationship between low moods and a program’s end. Instead, he says that when viewers feel lasting negativity, TV consumption might be acting as an anesthetic for a deeper psychological issue, like how some people with anxiety or depression drink alcohol. Put another way, the low mood already existed, and watching the series only masked it.

Why it’s so hard to say goodbye

There’s no reason to be worried if you get sad or annoyed with the ending of a series you adore—after all, Kong says, people do feel emotionally connected with and invested in fictional characters. For most people, the negative feelings should dissipate shortly. 

If you’re looking to perk up when a finale has you down, though, “the short, Band-Aid answer is to move on to another series,” Kong says. “The bigger-picture answer is to make the show less central in your life and wellbeing. It might be a red flag if you have no other interests beyond a show or other series.” 

For those who experience strong PSD, the sensation can last for weeks, Kottasz says. “It seems to be the case from the data that people who struggle with anxiety, depression, and loneliness may be more inclined to become really big fans,” she says, who in turn experience prolonged sadness. If that’s the case, it’s probably time to seek further help from therapists or other mental health specialists.

[Related: Understanding your emotions can help you manage your anxiety]

What makes PSD more unusual than feelings of nostalgia or other losses, Kottasz says, is that enthusiasts “do have an opportunity to get things back” by persuading creators to make reboots, revivals, or spinoffs. Precedent for this dates back to before electronic TVs were invented: Author Arthur Conan Doyle tried to kill off Sherlock Holmes for good in 1893, only to resurrect the consulting detective in the early 1900s. The BBC suggests it was the first revival of a character after fan outcry. 

Aficionados can engage in other ways. One is travel, mixing tourism with fandom to experience a franchise in real life. Think Lord of the Rings buffs who visit filming locations like “Mount Doom” in New Zealand, or Game of Thrones fanatics who tour Belfast and Dubrovnik. The pattern continues. On May 29, the Monday after Succession aired for the last time, fans flocked to New York City’s Battery Park, the scene of the series’s final shot.

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In an environment without light, or where sight is otherwise useless, some creatures have learned to thrive by sound. They rely on calls, clicks, and twitters to create a kind of map of their surroundings or pinpoint prey. That ability is called echolocation, and a simple way to understand how it works is to crack open the word itself. 

What is echolocation?

Imagine an echo that locates things. The sound hits an object and bounces back, relaying information about a target’s whereabouts or cues for navigation. When Harvard University zoologist Donald Griffin coined the word “echolocation” in the journal Science in 1944, he was describing how bats rely on sounds to “fly through the total darkness of caves without striking the walls or the jutting stalactites.”

In the decades since, scientists have identified many other animals that use echolocation, aka biosonar. For example, at least 16 species of birds echolocate, including swiftlets and nocturnal oilbirds, which roost deep in South America’s caves. Laura Kloepper, an expert in animal acoustics at the University of New Hampshire, calls this shared ability an example of convergent evolution, in which “you have two unrelated species evolve the same adaptive strategy.” 

How does echolocation work?

To find fish in deep waters, or avoid crashing in the inky night, whales and bats produce loud ultrasonic sounds at frequencies all the way up to 200 kilohertz. That is way beyond human hearing (most adults can’t perceive pitches above 17 kilohertz). 

[Related on PopSci+: 5 sounds not meant for the human ear]

Why do specialized echolocators use ultrasonic sound? “High-frequency sounds give really fine spatial resolution,” Kloepper explains. Hertz is a measure of the distance between each acoustic wave: The higher the hertz, the tighter the wave, and the smaller the detail captured by the vibration of energy in the air. If you were to echolocate in a room, a big, low-frequency wave might simply reflect off a wall, Kloepper says, while an echo from a higher-frequency sound could tell you where the doorway or even the knob was.

Echoes, if you know how to interpret them, are rich in information. As Kloepper explains it, when an animal with the ability hears a reflection, it examines that sound against an “internalized template” of the call it sent out. That comparison of echo versus signal can yield the distance to a target, the direction it might be traveling in, and even its material make-up.

Ultrasonic calls give another bats boost, too—they rely on next-level frequencies to find mates. Many species of moths hunted by bats have evolved ears attuned to these frequencies as a means of survival.

[Related: How fast is supersonic flight?]

To make ultrasound, a bat vibrates a specialized organ in its throat called a larynx. It’s not too different from how the human voice box works, except the bat produces a much higher frequency sound. Certain bat species then release the sound from their mouths, while others screech from the snout, using an elaborate nasal structure nicknamed a nose-leaf. 

Whales

Dolphins, orcas, and other toothed whales echolocate for the same reasons as bats do: to chase down tasty prey and navigate through darkness. But these aquatic mammals emit ultrasound in a completely different way. Inside whale heads, often close to their blowholes, sit lip-like flaps. When the animals push air across the flaps, the appendages vibrate, producing clicks. “It’s just like if you inflate a balloon and let all the air out of that balloon. It makes a pbbft noise,” Kloepper says. 

The curves of dolphin skulls propel that noise into fatty structures at the front of their heads, called melons. These, in turn, efficiently transmit vibrations in seawater. The waves bounce off prey or other objects, but the whales don’t rely on external ears to hear the echo (their ear canals are plugged up with wax). Instead, the vibrations are channeled via their jawbones, where sound is received by fat-filled cavities so thin that light can pass through them. The cavities are near the whales’ inner ears, which sense the echoing clicks. The process can reveal all sorts of details: where a fish is, where it’s going, and how fast it’s swimming.

Shrews

Shrews have sensitive whiskers but poor eyesight. To supplement their senses as they explore their forest and grassy meadow habitats, they might use a coarse form of echolocation, which Sophie von Merten, a mammalogist at the University of Lisbon in Portugal, calls “echo-orientation” or “echo-navigation.” This ability could “give them a hint that there is an obstacle coming,” she says, such as a fallen branch detected by the shrews’ twitters. Their bird-like sounds are faint, but audible to humans. 

The extent of shrew echo-navigation isn’t entirely clear. In a 2020 “experiment, von Merten and a colleague found that, when shrews are introduced to new environments, the wee mammals twitter more frequently. Von Merten says it’s likely they are sensing the unfamiliar location by these vocalizations, but another interpretation could be that the captive animals are stressed. That’s a hypothesis she doesn’t find very convincing, though her ongoing research will measure shrew stress, too.

Could there be other undiscovered creatures out there that echolocate? “I think it’s very likely,” Kloepper says. She adds that it’s hard to tell which animals beyond mammals and birds display the behavior, given “just how little we know about vocalizations of many cryptic species.”

Humans

Unlike bats, people aren’t born with the innate power of echolocation—but we can still make it work. In his original 1944 paper, Griffin discussed a, such as captains listening for echoes of ship horns against cliff faces, or those who are blind following the taps of their canes. 

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On Sunday, phone lines across the world will be their busiest—if we all remember to call our mothers. But we’re hardly the only creatures with good reason to celebrate our moms. We’ve searched the Popular Science archives to give you a roundup of stories featuring heroic mothers from across the animal world.

The moms who really give parenting their all

For the wriggling offspring of the Taita Hills caecilian, there’s no better taste than mom’s peeling skin. These land-dwelling, legless amphibians—found in the forests of southern Kenya—rip off the thick, protein-packed layer with gusto. By the time her kids are done feasting, mama caecilian will lose more than a tenth of her body weight.

Then there’s the mother desert spider, whose sacrifice for her brood goes more than skin-deep. Once the hatchlings emerge, she spits up her own meals to feed her children. Pretty tame, until the digestive enzymes that come up with her puke eat away at her insides. She’ll continue to feed and protect her young for the next two weeks while the enzymes from her stomach kill her from the inside out. But the baby spiders won’t let the corpse go to waste—they’ll gobble up what remains of mom before setting out on their own.

A whole lot of milk

Mammals might only feed their newborns breast milk, but that doesn’t mean they are any less impressive. Elusive orangutan mothers will breastfeed their children at night and under tree cover for up to eight years, well past the time their young is small enough to carry around. That’s the longest any wild animal nurses their young.

For the burrowing, venomous, shrew-looking mammal called a solenodon, children don’t get milk near mom’s chest. The offspring climb next to her butt, where her nipples are found, to breastfeed for several months.

Love to spare

Some mothers don’t raise their own biological children, or even members of their own species. At the end of the year, biologists on an American island in the North Pacific got really excited when a Short-tailed albatross couple had an egg in their nest. It would have been the fourth time that a chick of the endangered species was born on U.S. soil. But once the egg hatched and scientists peered into the nest, they didn’t find a little Short-tailed albatross. Instead, the couple had adopted an egg from a smaller, more common bird, the Black-footed albatross. The researchers weren’t too disappointed—fostering a foundling could be great practice for the first-time parents.

Sharks with virgin births

For certain species of fish, fathers are optional. At the Shedd Aquarium in Chicago, female zebra sharks reproduced by fertilizing their eggs with their own genetic material. That process, known as parthenogenesis, is typically a last-ditch move when males aren’t available. But the female sharks shared their enclosure with potential mates. “This changes what we think we know about parthenogenesis and why it occurs,” said Lise Watson, Shedd Aquarium’s assistant director of animal operations and habitats, to Popular Science.

Mouse moms teach parenting life skills

Young mice get pointers on how to parent from older mother mice. A study published in the journal Nature in 2021 reported a behavior called “shepherding,” in which mother mice pushed virgin female mice into a nest of crying mouse pups. It’s as though the mothers were urging the other mice to learn a lesson in babysitting: “It wasn’t violent or forceful or aggressive, but definitely like an experienced mom grabbing the older child by the hand and dragging them into the nursery,” study author and NYU professor Robert Froemke told Popular Science. 

Straight from the womb

Humans have a few weird quirks thanks to their moms, beyond the traits they might have inherited. If a mother eats strong flavors like garlic, vanilla, or mint while pregnant, their infant may be more gung-ho to try those foods later on. The microbes found in our guts are also from our moms. A lot of those bugs come straight from her birth canal. That’s just one more thing to thank your folks for when you give them a call this weekend.

This post has been updated. It was originally published on May 12, 2018.

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ANCIENT ROMANS were masters of concrete, fashioning concoctions of sand, water, and rock into long-lasting marvels. Bridges, stadiums, and other structures they built with the stuff still stand tall—even harbors and breakwaters that have been soaked by tides and storms for nearly 2,000 years. This substance, robust to the microscopic level, far outlives the modern material, which generally requires steel supports in salt water and is still likely to corrode within decades.

When the Roman Empire ended, so did its method of making marine concrete. But by following chemical clues within ancient architecture, today’s scientists have revived this technique. In recent years, researchers have only gotten better at understanding it, applying lessons from fields as diverse as archaeology, civil engineering, and volcanology. They have pulled tubes of the ancient substance from under the ocean. They have zapped it with X-rays to observe its microscopic minerals. Now they’ve mixed up their own industrial version.

In 2023, for the first time in nearly two millennia, Roman-style marine concrete will be tested on a coastline. Silica-X, a US-based company that specializes in experimental glass, plans to place four or five slabs into Long Island Sound beginning this summer. Unlike virtually all other concrete products made today, which are designed to resist their environments, these 2,600-pound samples will embrace their aquatic surroundings—and are expected to become stronger over time.

As water moves through the porous solid, the material’s minerals will dissolve, and new, strengthening compounds will form. “That is actually the secret of Roman concrete,” says University of Utah geology and geophysics research associate professor Marie D. Jackson, who is working on a reboot of the stuff with a $1.4 million grant from the Advanced Research Projects Agency–Energy, a federal program that supports early-stage technology research. 

Jackson has spent more than a decade investigating what happens when Roman concrete meets seawater. She is part of a team working alongside Silica-X; the prototypes destined to be dunked in the New York estuary are based on her recipe.

“One hundred percent, Marie is the most significant person” trying to understand and develop the substance, says her frequent collaborator, Google hardware developer Philip Brune. More than a decade ago, when Brune was a Ph.D. student, he and Jackson created the first of what they call Roman concrete analogues. After making a terrestrial type—similar to the basis of the Pantheon and Trajan’s Market—they switched to a marine variant.

Jackson has an application in mind for these historical replicants: guarding against the effects of climate change. The National Oceanic and Atmospheric Administration projects that by 2050, sea levels will rise by an average of 10 to 12 inches along American coasts. Modern concrete seawalls, which need to be replaced roughly every 30 years, already cover a substantial percentage of the US shoreline. If waves keep mounting, it will be necessary to find a more durable and sustainable option to reinforce our seaboards. 

The duality of concrete

Concrete’s ingredients are about as simple as a sugar cookie’s. Besides water and air, it requires a grainy material called aggregate, which may be sand, gravel, or crushed rock. The other necessity is cement, a mineral glue that holds the constituents together. Portland cement, invented in the mid-1800s in England, remains the basis for the majority of modern concrete formulas. This mix results in a consistently potent product. “You can make it on Mars with the same ingredients, and you know it will work,” says Admir Masic, associate professor of civil and environmental engineering and principal investigator at the Massachusetts Institute of Technology’s Concrete Sustainability Hub.

Portland cement production is the noxious part: Not only is it thirsty for fresh water and energy, it also releases loads of carbon dioxide. The manufacturing process is responsible for 7 to 8 percent of worldwide CO₂ emissions, according to Sabbie Miller, a civil and environmental engineering assistant professor at the University of California, Davis. If the global concrete industry were a nation, its greenhouse gas footprint would be the third biggest on the planet, after those of China and the US. 

The concrete sector is aware of its product’s environmental legacy and is willing to work toward change, Miller says. Global construction conglomerate HeidelbergCement, for one, announced in 2021 that it would construct the first carbon-neutral cement plant by 2030, a facility that would capture greenhouse gases and lock them up in bedrock below the sea. Other types of concrete in development are designed to lock up pollution within the material itself. Miller, who is working on techniques to turn carbon into a solid, storable mineral, says these are “very much early-days, we’ll-see-if-it-works technologies.”

Making concrete as the Romans did should reduce troublesome emissions, researchers say, in large part because this substance won’t need to be replaced frequently. Yet the ancient process doesn’t yield quite as much compressive strength—this resource won’t hold up super-tall buildings or heavily traveled bridges. In the concrete heart of Manhattan, “We will not use Roman-inspired material,” says Masic, who co-authored a paper with Jackson and two others on reactions within the building materials in the Roman tomb of Caecilia Metella and is an inventor of what he calls a “self-healing” substance. Rather, he says, the timeless concoction could be fashioned into roads that resist wear, walls that withstand waves, and vaults that confine nuclear waste.

What Roman-style concrete does best is survive, aided by its ability to repair itself within days. “This material has phenomenal durability,” Brune says. “Nothing else that you find in the built environment lasts with as much integrity and fidelity.” A key ingredient that gives it this ability lies in the sand-like pozzolan of Pozzuoli, Italy.

From fire to the sea

Jackson did not set out to unlock the secrets of Roman concrete. Drawn to volcanology and rock mechanics, she studied Hawaii’s Mauna Loa in the late ’80s and early ’90s. In 1995, she spent a year in Rome with her family, living near the ruins of the Circus Maximus, once a huge chariot-racing stadium. While there, she became fascinated with the volcanic rock incorporated into the city’s celebrated classical architecture.

Roman concrete has been the subject of intense scholarship—structures that persist for thousands of years tend to attract attention. But Jackson, with her geologist’s eye, saw something powerful below the surface. “It is very difficult to understand this material unless one understands volcanic rocks,” she says. In her analysis, Jackson focused on tephra, particles spit out in a volcanic eruption, and tuff, the rock that forms when tephra firms up. 

Her first paper about Roman building materials, a collaboration with four other scientists, was published in the journal Archaeometry in 2005. The group described seven deposits where ancient builders had collected tuff and stones. These were products of explosive eruptions from two volcanoes north and south of Rome. By the first century BCE, Roman architects had recognized the resilience of these rocks and had begun to place them in what Jackson notes were “strategic positions” around the city.

While she examined materials in the Eternal City, others were separately scouring the sea. A trio of scholars and scuba divers—classical archaeologists Robert L. Hohlfelder and John Oleson, and London-based architect Christopher Brandon—launched the Roman Maritime Concrete Study in 2001. Over the next several years, they collected dozens of core samples from Egypt, Greece, Italy, Israel, and Turkey, taken from 10 Roman harbor sites and one piscina, a seaside tank for corralling edible fish.

Some of the locations they inspected were immense structures: At Caesarea Palaestinae, a port city built between 22 and 10 BCE during the reign of King Herod, Romans created a harbor from an estimated 20,000 metric tons of volcanic ash. 

To look inside the ruins, the archaeologists needed heavy machinery. “You used to whack some pieces off the outside of a big, maybe 400-cubic-meter lump of concrete on the ocean floor,” says Oleson, a University of Victoria professor emeritus. But that approach has flaws. The surface is already decayed from sea growth, so whatever breaks off might not represent what’s deeper inside. “You’ve also been whacking on it with a hammer,” he says, which can foul the opportunity to measure its material strength.

The project required a more precise, piercing touch. A cement company in Italy, Italcementi, provided funding and helped get the three men a specialized hydraulic coring rig. Diving beneath the Mediterranean, they spent hours drilling, extracting cylindrical cores up to 20 feet long. “It was difficult,” Oleson says. “In places like Alexandria, the visibility—because of all the things you don’t want to think about—was less than your arm length.”

That effort paid off. No one had been able to look at the layers within the submerged structures before. The opinion at the time was that the concrete must have been extra strong to last for thousands of years in seawater. But that wasn’t the case, Oleson and his colleagues found: “In modern engineering terms, it’s quite weak,” he says. What it was, though, was remarkably consistent in its volcanic elements. Oleson theorizes that grain ships used Neapolitan pozzolan as ballast, ferrying it to work sites hundreds of miles from its source.

In 2007, the trio’s presentation on seawater concrete won an award at the Archaeological Institute of America’s annual meeting. “I was standing there, bathing in the glory, and this short, excitable woman came up and started talking to me,” Oleson recalls. The stranger was Jackson, who Oleson says launched into a detailed explanation of the rare crystal minerals she had observed within Roman architectural concrete. Oleson, for his part, had never taken a college chemistry course, but he recognized a kindred spirit—and that this geologist had expertise his group needed. 

They gave Jackson access to the maritime samples. And when she peered inside, she found chemical laboratories on a nanometer scale.

Reactions in the rock

In his first-century work Natural History, Roman author Pliny the Elder wrote of a dust that “as soon as it comes into contact with the waves of the sea and is submerged, becomes a single stone mass, impregnable to the waves and every day stronger.” How precisely these wet grains—the pozzolan—became ever stronger would not be revealed for almost 2,000 years.

When Jackson investigated the core samples Oleson and his colleagues had obtained, she spotted some of the same features she’d seen in the architectural concrete in metropolitan Rome. But in the sunken stuff, she also saw what she labeled mineral cycling: a looping reaction in which compounds formed, dissolved, and formed new ones.

To make concrete, Romans mixed tephra with hydrated lime. That accelerates the production of a mineral glue called calcium aluminum silicate hydrate, or C-A-S-H. (The backbone of unadulterated modern concrete, C-S-H, is a similar binder.) This happens within the first months of installation, Jackson says. Within five to 10 years, the material composition changes again, consuming all the hydrated lime through a kind of microscopic interior remodeling. By then, percolating fluid “begins to really make a difference” as it produces long-lasting, cementlike minerals within.

Jackson and a team of scientists used sophisticated microscope and X-ray techniques, including work done at the Lawrence Berkeley National Laboratory’s Advanced Light Source, to look at these powerful but teeny crystals. “We were able to show systematically that Roman seawater concrete had continued to change over time,” she says. Within each pore of the concrete, seawater had reacted with glass or crystal compounds. In particular, she found stiff, riblike plates of a rare mineral known as aluminous tobermorite, which probably help prevent fractures, as she and her colleagues wrote in a 2017 paper in the journal American Mineralogist.

The ocean itself plays a vital role. Roman fabricators made their marine concrete mixtures with seawater, and its salts became part of the mineral structure—sodium, chlorine, and other ions helped activate the tephra-lime reaction. Once the concrete was in the tides, as fluid slowly percolated through the hulking edifices, life flourished on the facades. Worms made tubes and other invertebrates sprouted shells.

Modern reinforced concrete, meanwhile, needs a high pH to preserve the steel rebar within, which means its surface is less friendly to living things. Once it is cast, after about 28 days of hardening and curing, it is near its maximum sturdiness, Brune notes. (Attempts are underway to give newer kinds of concrete the ability to restore themselves, such as infusing the material with bacterial spores that create limestone.)

Specifically, concrete using Portland cement is as brittle as it is strong. Under too much strain, it cracks, sometimes with a sharp snap that propagates and causes wide-scale failure. “The ability of the material to carry further loads, it’s gone. It’s fractured,” Jackson says. 

Roman concrete breaks differently. Brune and Jackson have tested their analogues under strain, creating semicircles out of the blend and pressing them to the cracking point. They observed that unlike extremely inflexible substances that will fail and essentially split into halves, Roman concrete displaces the strain over many small fractures, without necessarily losing its overall integrity. “Roman concrete-style materials respond really well to that kind of cracking,” Brune says, adding that this feature could explain why the age-old recipe has endured so long despite earthquakes and the churn of aquatic environments.

White clumps of lime found in Roman concrete can also keep it robust, as Masic and fellow MIT scientists reported in a Science Advances paper in January. In lab experiments, the team drained water through cracked concrete cylinders for 30 days. Water continuously flowed through broken samples of typical concrete. But in concrete with added lime gobs, calcite crystallized to fill the gaps. 

Jackson and Brune have observed similar self-restoring abilities in their marine concrete replicas. In to-be-published experiments funded by the Advanced Research Projects Agency-Energy grant, they again cracked semicircles of the concoction. When they placed the damaged arcs in containers of seawater, chemical reactions resumed—new glue accumulated in the fractures. This, Jackson says, is concrete that self-repairs.

New trials, new island

As 2023 surges on, Roman-style concrete will venture further than ever before. US Army Corps of Engineers research geologist Charles Weiss, who studies concrete and other structural materials, has submitted a proposal to try out Jackson’s formula. If the Vicksburg, Mississippi, military lab receives the funds—“Working for the government, nothing is for sure,” he says—Corps researchers will cast the material and place it in a body of water.

Elsewhere, another federal project’s failure may have helped Jackson’s creation along. In 2018 in South Carolina, at the Department of Energy’s Savannah River National Laboratory, scientists were trying to make a product that could safely store radioactive garbage.

The national lab wanted to create foam glass, a type of bubble-filled substance meant to be inactive, and contracted the Silica-X team to help. They weren’t successful. The mixture kept reacting with its surroundings—a problem because if radioactive waste receptacles dissolve, they can release unstable particles. But what’s bad for nuclear trash is good for seawalls designed to respond to their environs. Glass designers at the lab recognized this potential and connected Jackson with the company.

Despite the growing interest in Roman-style concrete, it is neither feasible nor sustainable to mine industrial amounts of pozzolan from Naples. Instead, Philip Galland, Silica-X’s chief executive, says its production process digs into nonnuclear US waste streams to obtain silica, which is then transformed into synthetic tephra. That will be the basis for the upcoming Long Island Sound field test, Galland says, in an “area where it can offer shoreline resilience.”

Silica-X plans to assess the 3.5-foot cubes’ durability over two years. Along with its partners—the New York Department of Environmental Conservation and Alfred University, home to an influential ceramics college—the company will analyze the material’s potential as a storm-surge barrier and how it performs as a habitat for microbes and other local marine life.

At the same time, Jackson has returned to her original subject matter: volcanoes. She is the principal investigator of a project to study Surtsey, a tiny volcanic island off of Iceland that’s just 60 years old. A UNESCO World Heritage site, it emerged from the Atlantic in sprays of smoke and lava from 1963 to 1967. “I remember when it first erupted,” Jackson says, “because my dad came home from work and told us that there was a baby volcano erupting.”

At Surtsey, scientists have found microbial life in basalt rocks previously untouched by humans. (Aside from research teams who arrive by boat or helicopter, visitors are banned from the volcano.) They have drilled to the seafloor, through stone that is still hot years after the last eruption, and examined the tephra there. As it slumbers, Jackson believes this place can reveal what happened in the early years of submerged Roman concrete. 

What she knows about the material has been gleaned from stuff that’s aged for thousands of years underwater. Although the young terrain is an imperfect replica of the coveted ancient ingredient—the fluids there aren’t quite the same as what percolated through the Roman structures—Jackson says she has already spied some similar geochemical processes. The ash and seawater around the volcano offer a parallel to the early reactions that gave a great civilization its building blocks. This is a living laboratory that could teach us Roman concrete’s art of change, witnessed on a scale as massive as a new island or as tiny as minerals morphing across millennia. If all goes well, the modern version of this powerful invention will outlast its makers just the same.

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ON A FOGGY midsummer morning 54 miles northwest of Santa Barbara, California, SpaceX engineers hustled through a ritual they’d been through before. They loaded a Falcon 9 rocket with tens of thousands of gallons of kerosene and supercold liquid oxygen, a propellant combo that brought the craft’s nine Merlin engines roaring to life with 1.7 million pounds of thrust. Soon after, the machine shot through the stratosphere, ready to dispatch 46 of the company’s Starlink internet satellites into low Earth orbit. But the rocket made another delivery too: a trail of sooty particles that lingered over the Pacific hours after blastoff.

The launch was the company’s 32nd of 2022, maintaining its current pace of firing off close to one rocket per week. With a record number of rides shuttling equipment, astronauts, and über-rich tourists to and from Earth, the high skies have never been busier. Between government programs like China’s Long March and private shots like SpaceX’s Crew Dragon, the world tallied some 130 successful launches in 2021 and is on pace to finish 2022 with even more. Many trips, however, spew tiny bits of matter into the stratosphere, an area that hasn’t seen much pollution firsthand yet.

Climate scientists are still working to fully understand how rocket residue affects the planet’s UV shield. But even if they find warning signs, some organization or authority figure would have to step up to establish emission standards for the industry. In the meantime, a few aerospace companies are exploring sustainable alternatives, like biofuels, to power their far-flying systems.

The increasing frequency of launches has researchers like Martin N. Ross, an atmospheric physicist and project engineer at the Aerospace Corporation, a nonprofit research center in California, worried about the future of the stratosphere—and the world. Predictions for rocket traffic in the coming decades point dramatically up, like a Falcon 9 on a pad. Should the sun heat up enough of the particles from the fuel trails, as some computer models suggest it will, space travel could become a significant driver of climate change. “This is not a theoretical concern,” Ross says.

Unless you are reading this while floating aboard the International Space Station, you are breathing air from the troposphere—the bottommost band of the Earth’s atmosphere, which extends upward for several miles. The layer just above that, the stratosphere, sits anywhere from 6 to 31 miles above sea level and is deathly dull by comparison: There are barely any clouds there, so it doesn’t rain. The air is thin and freezing and contains ozone, an oxygen-based gas that protects all life from solar radiation but is toxic to the lungs.

Most greenhouse gases, including the 900 million tons of carbon dioxide produced by the aviation industry each year, trap heat in the troposphere. But rockets rip their vapors at higher altitudes, making them the single direct source of emissions in the upper stratosphere.

Acid in the sky

The stratosphere was people-free until 1931, when Swiss physicist Auguste Piccard and his aide floated nearly 10 miles up, and back down, with a 500,000-cubic-foot hydrogen balloon. They were the first of many. By the 1960s, the US and Soviet space programs were regularly shooting rockets to the edge of the sky.

As astronaut and cosmonaut programs evolved during the Cold War, so did climate change research—especially studies of carbon dioxide pollution and atmospheric degradation. In the 1970s, NASA’s space shuttle program piqued the interest of atmospheric chemists like Ralph Cicerone and Richard Stolarski, who then attempted some of the first investigations into stratospheric rocket exhaust. The shuttle’s solid engines used a crystalline compound called ammonium perchlorate, which releases hydrochloric acid as a byproduct. Chlorine is highly destructive to ozone—the Environmental Protection Agency estimates a lone atom can break down tens of thousands of molecules of the atmospheric gas.

In a June 1973 report to NASA, Cicerone, Stolarski, and their colleagues calculated that 100 shuttle launches a year would produce “quite small” amounts of chlorine-containing compounds. But they warned that these chemicals could build up over time. Cicerone and Stolarski ultimately focused their attention on volcanic eruptions, because those belches represented larger and more dramatic releases of chlorine.

In the 1980s, British meteorologists revealed that the ozone layer in the Antarctic stratosphere was thinning. They identified the culprit as chlorine from aerosol spray cans and to O3-munching chemicals called chlorofluorocarbons from other human-made sources. That hole began to heal only after the 1987 Montreal Protocol, the first international agreement ever ratified by every member state of the United Nations. It phased out the use of CFCs, setting the atmosphere on a decades-long path to recovery.

In the wake of that treaty, “Anything that emitted chlorine was under suspicion,” Ross says. But it remained unclear whether rocket emissions too could alter the ozone layer.

For the following two decades, the US Air Force enlisted the Aerospace Corporation and atmospheric scientists like Darin Toohey, now a University of Colorado Boulder professor, to study the chemical composition of rocket exhaust. Using NASA’s WB-57 aircraft, a jet bomber able to fly 11 miles high and retrofitted for scientific observations, teams directly sampled emissions from American launch vehicles including Titan, Athena, and Delta into the early 2000s.

Freshly collected material from the plumes gave the researchers a firmer grasp on the ways solid propellant interacted with air. For instance, they examined the particles that were expelled when shuttle boosters burned aluminum powder as fuel—and how those bits reacted to ozone. The effect wasn’t as severe as they had feared, Ross says. Though the plumes depleted nearby ozone within the first hour after a launch, the layer was quickly restored after the emissions diffused.

Meanwhile, at the turn of the 21st century, blastoffs were decreasing in the US and Russia. After the space shuttle Columbia disintegrated on reentry in 2003, killing its seven-person crew, NASA suspended other flights in the program for two years. Missions using the WB-57 aircraft to observe exhaust came to an end in 2005. Six years later, NASA officially retired the shuttle system.

New rockets, more soot

When SpaceX sent its first liquid-fueled rocket into orbit in 2008, it set the stage for more privately developed spaceflights. But the chemical it pumped into its marquee machines wasn’t anything new. A refined version of kerosene, Rocket Propellant-1 or RP-1, has powered generations of rockets, including the first-stage engines of the spaceships that ferried Apollo astronauts to the moon. It was well known and relatively cheap.

Sensing an aerospace trend, Ross, Toohey, and their colleague Michael Mills calculated what emissions would be produced by a fleet of similarly hydrocarbon-powered rockets anywhere between the Earth’s surface and 90 miles aloft. Their predictions, which they published in 2010 in the journal Geophysical Research Letters, turned up something unexpected: an emissions signature full of black carbon, the same contaminant belched by poorly tuned diesel engines on the ground. “It seemed to have a disproportionate impact on the upper atmosphere,” Toohey says.

Those dark particles are “very, very good at absorbing the sun’s radiation,” adds Eloise Marais, an atmospheric chemist at University College London. Think of how you heat up faster on a hot summer day while wearing a black shirt rather than a white one, and you get the idea.

Near the ground, rain or other precipitation will flush dark carbon out of the air. But in the stratosphere, above rain clouds, soot sticks around. “As soon as we start to put things in that layer of the atmosphere, their impact is much greater, because it’s considerably cleaner up there than it is down here,” Marais says. In other words, the pristineness of the stratosphere makes it more vulnerable to the sun’s searing rays.

Black carbon particles can persist for about two years in the stratosphere before gravity drags them back down to the ground. They also heat up as they wait: In a study published this June in the journal Earth’s Future, Marais and her colleagues calculated that soot from rockets is about 500 times more efficient at warming the air than that from planes or emitters on the surface.

Another recent model run by Ross and Christopher Maloney, a research scientist at the National Oceanic and Atmospheric Administration Chemical Sciences Laboratory, came to a similar conclusion about the dark stuff’s impact on climate change. Should space traffic increase tenfold within the next two decades, the stratosphere will warm by about 3 degrees Fahrenheit, they predict.

That uptick is enough that “stratospheric dynamics [will] begin to shift,” Maloney says. Currents carry naturally produced ozone from hotter tropical regions toward cooler poles. If rockets scorch a pool of air above the Northern Hemisphere, where most launches take place, that warm-to-cold path could be disturbed—disrupting the circulation that ferries fresh O3 northward. The upshot: a thinner ozone layer at the higher latitudes and a toastier stratosphere overall.

Sizing up old launches can help clear up some of the gray areas in this process. In a paper published this July in the journal Physics of Fluids, a pair of researchers at the University of Nicosia in Cyprus simulated the plume from a SpaceX Falcon 9 rocket from 2016. According to their model, in the first 2.75 minutes of flight, the craft generated 116 tons of carbon dioxide, which is equivalent to a year’s worth of emissions from about 70 cars.

Toohey sees these projections as validation of the black carbon concerns he raised more than a decade ago—but thinks they’re not as compelling as direct observations would be. There has been “basically no progress, except additional model studies, telling us the original hypothesis was correct,” he says. What’s needed, he adds, is detection in the style of the earlier WB-57 missions. For example, spectrometers planted on the sides of spaceships could measure black carbon.

Policy is another limiting factor. The International Air Transport Association, an influential trade organization, has set carbon-neutral goals for airlines for 2050, but there is no comparable target for space—in part because there is no equivalent leader in the industry or regulatory body like the Federal Aviation Administration. “We don’t have an agreed-upon way to measure what rocket engines are doing to the environment,” Ross says.

While there are newer fuels out there, there’s no good way to determine how green they are. Even the one that burns cleanest, hydrogen, requires extra energy to be refined to its pure molecular form from methane or water. “The picture is very complex, as all propellants have environmental impact,” says Stephen Heister, who studies aerospace propulsion at Purdue University.

Atmospheric scientists say solutions to preserve the stratosphere must be developed collaboratively, as with the unified front that made the Montreal Protocol a juggernaut. “The way to deal with it is to start getting people with common interests together,” Toohey says, to find a sustainable path to space before lasting damage is done.

Photo credits for lead image: Left to right, top to bottom: Patrick T. Fallon/Getty Images; Wang Jiangbo/Xinhua/Getty Images; Zheng Bin/Xinhua/Getty Images; Yang Guanyu/Xinhua/Getty Images; Cai Yang/Xinhua/Getty Images; Wang Jiangbo/Xinhua/Getty Images; Wang Jiangbo/Xinhua/Getty Images; Korea Aerospace Research Institute/Getty Images; SOPA Images Ltd./Alamy (2); Jonathan Newton/The Washington Post/Getty Images; GeoPix/NASA/Joel Kowsky/Alamy; Wang Jiangbo/Xinhua/Getty Images; Paul Hennessy/Anadolu Agency/Getty Images; Zheng Bin/Xinhua/Getty Images

This story originally appeared in the High Issue of Popular Science. Read more PopSci+ stories.

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A respiratory virus that commonly infects infants and young children—and can be harmful to older adults, too—is circulating across the US. Cases of respiratory syncytial virus, or RSV, rose higher and earlier in the year than what’s expected of a usual winter-season spike. By the start of November, the virus had swamped pediatric wards at such levels that clinicians told The New York Times it brought to mind the COVID pandemic’s first surge. 

Most children who catch RSV have mild cold-like symptoms, but a fraction become very ill. By one global estimate, the virus is responsible for about 1 in 28 deaths among children four weeks to six months old.

No US-approved vaccines exist for this pathogen. But, just as newly made immunizations drove down deaths and hospitalizations from COVID, jabs for RSV would be a welcome tool, able to forestall future outbreaks. Pharma giant Pfizer, for instance, recently announced it has promising data in early trials for its protein-based formulation. Pfizer says it plans to submit the single-dose shot to the Food and Drug Administration for approval by the end of 2022.

The company says its treatment protects infants in the first six months after birth—but the injections aren’t meant for babies. Instead, people who are pregnant would get them in the late second or third trimester; the immune defenses their bodies make are passed on through the placenta to fetuses, and, later, by breast milk to infants. In this way, the vaccine creates RSV-specific antibodies inside a parent, which then flow through the natural pathways that connect a mother to child. This powerful combination could be part of a growing approach to combat numerous viruses able to harm mothers and their newborns.

[Related: Improving your baby’s bone health starts in the womb]

The placenta—the organ that temporarily exists to serve the unborn in the uterus—is a biological mash-up of a scuba tank and room service: It supplies oxygen and food to a growing fetus. It also actively delivers maternal antibodies, especially near the end of a term. “A full-term baby has a robust load of antibodies that were bequeathed from the mom during pregnancy,” says Milagritos Tapia, a pediatrics professor at the University of Maryland School of Medicine. 

In studies on immunological protection during pregnancy, antibodies—proteins that the immune system uses to tag and flag invaders—get the lion’s share of attention. Vaccines go the extra mile to trigger a person’s body to generate fresh antibodies against a specific virus. There are other defenses that get passed on to infants, but those aren’t as easy to measure. “The reason that we talk about antibodies is they’re easier to understand and explain,” Tapia says. 

Infant immune systems

Infant immunity has many nuances. Young children, as the maxim among immunologists goes, aren’t simply small adults whose bodies respond proportionately less to a vaccine. Even after they’re born, their immune systems keep developing the ability to produce antibodies. The first vaccine shots that babies receive don’t evoke as strong a response as later ones, which is why infant immunizations against diseases like hepatitis B or polio come as a three-dose series.

[Related: Why are kids’ immune systems different from adults’?]

The protection babies get during pregnancy or from breast milk, then, is a stop-gap measure. Luckily, those temporary guardians are effective. There’s clear evidence that antibodies against RSV are efficiently passed onto children during pregnancy.

In a first-of-its-kind study looking at RSV protections from maternal antibodies, Tapia and her colleagues followed nearly 600 mother-infant pairs in Mali. They measured the levels of antibodies for RSV in infants at birth and in umbilical cord blood; higher levels were associated with significantly decreased odds of newborns having a case of RSV in the first three months of their lives, as shown in a study published in the journal Clinical Infectious Diseases in 2020. Put another way, the more neutralizers the babies emerged with, the better defended they were in their vulnerable early stages.

“It was what we had expected to see,” Tapia says. “Sometimes you have to prove what you think.” It also strongly suggests that “a vaccine against RSV delivered to pregnant women” would protect infants against infection from the virus in early infancy, as she and her co-authors write in the paper. Though the researchers didn’t study immunizations directly, showing that a baby born with lots of antibodies—which come from the parent during pregnancy—is a good sign antibodies generated from a maternal vaccine also end up defending the child after birth.

Vaccines given to pregnant people have precedent. The American College of Obstructions and Gynecologists recommends that those who are pregnant get the Tdap vaccine—so nicknamed because it protects against tetanus, diphtheria, and pertussis—between 27 weeks and 36 weeks of gestation, and preferentially on the earlier side of that window. The Centers for Disease Control and Prevention also advocates that pregnant women should get a flu vaccine (by injection, not the live form of the virus that comes in a nasal spray). Babies born to those who had maternal influenza vaccines are less likely to catch the flu in their first six months of life, studies have shown, too.

COVID connections

There is speculation that COVID quarantines may be responsible for the subsequent surge in RSV cases because pregnant people encountered the older virus less. 

“As lactating mothers were less exposed to common viruses during the COVID-19 pandemic due to preventative measures,” it’s expected that “the maternal immune system was not boosted, resulting in a lower production of antibodies,” write physicians Hannah Juncker and Britt van Keulen—a PhD student and postdoctoral researcher, respectively, at the Dutch Human Milk bank—in a joint email to Popular Science. Junker and Britt are co-authors of a study, published in June in the journal Microbiology Spectrum, that found antibodies against RSV in mothers’ milk decreased six months into 2020 lockdowns, shrinking by a factor of 1.7.

Fewer antibodies in human milk “led to a lower protection of the breastfed infant,” they write, resulting in a “massive epidemic” of RSV in the Netherlands in summer 2020.

If this hypothesis—that COVID protections are a contributor to RSV outbreaks—bears out, the vulnerability comes on quickly. “It’s one or two seasons of people missing their RSV exposure and, ka-bam!, here we are now,” Tapia says. “It seems like we need to be regularly exposed to these respiratory viruses.” 

The current US surge, two years after the Netherlands spike, highlights RSV as a global disease. For children younger than six months, the virus is the top cause of pneumonia worldwide. The vast majority of related deaths occur in low- to middle-income countries. 

“Not only are more babies getting sick” with RSV in developing nations, “but there are fewer resources to support them through their illness,” Tapia says; some hospitals may lack the breathing machines the sickest babies need. Even minor cases, which might cause a caregiver in the US to take a sick day, can have severe consequences in resource-poor nations where, Tapia points out, a parent missing a day’s work means their family goes hungry. The COVID pandemic has shown that access to vaccines can be starkly unequal; for an RSV shot to help on the global scale, it will have to boost the immune systems of infants in the poorest countries, too.  

As Pfizer continues to test its vaccine for safety and effectiveness, there are ways for pregnant people and those with children to protect themselves against RSV. “It’s the same things that kept COVID away,” Tapia says: Washing hands, wearing masks in crowded places, and staying clear of people who are sick or have symptoms are all ways to stay healthy in these virus-heavy times.

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With Artemis I now well underway, NASA is ready to dive into lunar exploration like never before. The game plan includes new tools, new experiments, and new landing sites, all leading up to a new generation of astronauts walking on the moon again.

But modern missions are only possible with the regolith-breaking research of the past, including decades of trial and error by NASA and other space agencies to get us closer and closer to Earth’s satellite. While Apollo might get most of the credit, there were plenty of attempts before the Saturn V rocket made it to the launch pad—and plenty of successes after the program was retired. Here’s what we’ve learned from some of those moonshots.

[Related: Is it finally time for a permanent moon base?]

Pioneer 0 (Able 1)

August 1958

The US Air Force was the first group from any nation to attempt to launch a rocket beyond Earth’s orbit and to the moon. It failed catastrophically: The booster carrying the probe exploded barely a minute after blastoff. Thankfully, the craft was uncrewed and was carrying relatively crude astronomy gear. NASA was created just a few months later. The Air Force ran its space ballistics programs under many different name though the 2000s, until the US government finally established a new military branch called Space Force.

Luna 1

January 1959

The USSR edged out the US in the 1950’s by successfully launching a lunar aircraft—that just kept going. The Soviet machine was essentially a silver ball studded with antennas, but lacking any kind of engine. While it was apparently designed to smash into the moon, it missed the satellite by about 1.5 times the lunar diameter and wound up orbiting the sun instead. That in itself was a milestone first.

Luna 2

September 1959

Luna 2 was successful where Luna 1 failed: The USSR smashed an uncrewed metal sphere into the moon, making it the first time anyone landed anything on the lunar surface. It was also the first time a human-made object touched something else in the cosmos. The mission’s precise final destination isn’t known, but it was somewhere near the northern Palus Putredinis region (which translates to “marsh of decay”), famous for hosting Apollo 15 in 1971.

Ranger 7

July 1964

This space probe, made at the Jet Propulsion Laboratory, which had recently pivoted to robotic extraterrestrial craft, was NASA’s first success at a lunar impact mission—after 13 straight failures. Before crashing (on purpose) into the moon’s Sea of Clouds plains, the probe took more than 4,300 photos of the lunar surface. The images were used to identify future landing sites for Apollo astronauts.

Luna 9

February 1966

When the USSR’s automatic lunar station touched down on the moon, it was the first artificial object to survive its visit. Airbags helped cushion its impact near a 82-foot-deep crater, though it still bounced around a fair bit before stabilizing. Over the next three days, the craft sent back images through its TV camera system, which were later stitched together into panoramic views. The first “soft landing” on another world was followed shortly by Luna 10, which was the first successful lunar orbiter.

Zond 5

September 1968

The first living things to travel around the moon were the two Russian steppe tortoises (and some worms) aboard a Soyuz capsule that circled the satellite for six days. The unnamed reptiles survived the journey, splashing down in the Indian Ocean before being retrieved by Soviet rescue vehicles. Since then, we’ve launched dogs, an “astrochimp,” and more benignly, baby bobtail squid into space.

Apollo 8

December 1968

Not long after the tortoise brigade, NASA’s Apollo 8 mission put the first people, American or otherwise, in lunar orbit. Frank Borman, James Lovell, and William Anders spent Christmas Eve flying around the moon 10 times in a 13-foot-wide capsule. Anders also famously took the photo “Earthrise” on the trip.

Apollo 11

July 1969

The Apollo missions progressed in quick succession, with the climax being the first steps on the moon. Astronauts Neil Armstrong, Buzz Aldrin, and Michael Collins logged some choice quotes as they made history in a voyage that was documented down to the last heartbeat. (Fun fact: Because NASA didn’t know whether there were microbes on the moon, the crew had to be quarantined for three weeks after their return.)

Chandrayaan-1

October 2008

India’s first deep-space mission made a big splash. The lunar probe, which kicked an ambitious new program into gear, carried NASA’s Moon Minerology Mapper, which, as a set of 2009 Science papers described, confirmed there were water molecules locked in our neighbor’s craters. Chandrayaan’s engineers lost contact with the machine 10 months into its orbital journey, following a sensor failure that caused it to overheat and killed its power supply. By then, though, the mission had completed 95 percent of its research objectives.

Chang’e 4

December 2018

The Chinese National Space Administration’s lander Chang’e 4 was the first craft to land on the moon’s far side. It touched down in a basalt crater in January 2019 and delivered a small rover, Yutu-2, that’s still exploring to this day. It also had some other special cargo: a cotton seedling that successfully germinated in a chamber on the moon, the first and only plant to do so.

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About a decade ago, a 25-year-old Oregon man named Rob Summers walked again, taking steps on a treadmill six years after a drunk driver paralyzed him from the chest down. Summers, as Popular Science reported in 2011, was among the first patients with paralysis to receive an experimental electrical device: a stimulator that restored his ability to walk after two years of training. 

Placing an electrode array in patients’ lower backs, researchers have found, can activate spinal circuitry in a way that repairs motor function. The needle-like implants deliver electricity directly to specific locations along the backbone. Exactly why these pulses restore walking, though, is something of a neurological mystery. A spinal cord injury can interrupt the connection between a person’s extremities and their brains, leaving them unable to move their limbs in some cases. For stimulation to work then, cells within the spinal cord’s own nerve system must reorganize in a way to process electrical signals, as the scientists leading these trials have hypothesized. If doctors can pinpoint how and why, they could apply the therapy to more kinds of paralysis, and someday, maybe even regenerate damaged cells.

Gaps in neuroscience

In recent years, clinicians have continued to test spinal implants, in some cases refining the stimulators’ design. A study published on November 9 in the journal Nature provides a clue to what changes in the vertebrae: Signals from the electrodes can excite a certain type of neuron in the nerves surrounding the spinal cord, as an international team of scientists found by examining mice. They note these neurons typically aren’t needed to help the rodents take steps—the cells only become activated for walking after the electrical stimulation treatment. The study also confirmed that the stimulators restored walking in nine people with chronic paralysis, whose spines were imaged via PET scan to measure cellular activity. 

Identifying these cells “is an important step towards gaining a better understanding of how the spinal cord controls movement” and how this therapy “improves walking function after a severe spinal cord injury,” says Ursula Hofstöetter, a professor at the Medical University of Vienna who studies the way our bodies control locomotion, but wasn’t involved in the new report. It might one day lead to drugs that target these neurons, too, she adds. 

Mapping the cells that become activated and the genes they express—in the case of the mouse model, the researchers pinpointed a gene in the neurons called Vsx2—can show scientists how spinal cords reorganize cell networks after injury. That’s the principle behind epidural electrical stimulation, the technique that helped Summers walk, as well as nine patients in the new study. (“Epidural” refers to the space between the spinal cord and backbone; it’s perhaps best known as the site where anesthesiologists may administer numbing medication to people in labor.)  

[Related: New implant helps patient spell out entire sentences using only brain signals]

An unusual pattern occurs in the spinal nerve systems of patients who receive this stimulation: Their movement may improve even when the stimulators are turned off. What’s more, the overall level of cell activity in the spinal networks appears to decrease, says study author Jordan Squair, a postdoctoral scientist at the Swiss Federal Institute of Technology. Essentially, he says, the spines are becoming “more efficient”—unlike at the start of rehabilitation, they no longer need to activate a whole bunch of cells to create movement. 

That led Squair and his colleagues to investigate which neurons wind up taking the lead.  After a spinal cord injury, healthy circuits become disrupted, says Michael Oh, a neurosurgeon at the University of California, Irvine. The activated cells identified in this study are probably intermediaries between the neurons responsible for sensation and those that govern motion, smoothing out that disorganization and streamlining firing circuits.

Squair believes these kinds of neurons govern more functions than walking, too. A 2021 Nature paper by many of the same authors as the new research showed epidural electrical stimulation can help regulate the blood pressure of patients with paralysis.

In case studies, epidural electrical stimulation has been so successful, Hofstöetter says, the recovery for people living with severe spinal cord injury reached levels “that were previously deemed impossible.”

What’s in a stimulator

In most instances, including for six patients in the more recent Nature study, the implanted electrode arrays were repurposed from machines originally designed to treat neuropathic pain. But three people in this report had an experimental device, under commercial development by Onward Medical and based on research by the Swiss company NeuroRestore, of which Squair is a part. 

Because this device was tailored to help revive function in patients with paralysis, Squair says the stimulator can restore sensation more quickly than older implants. “To activate the muscles that you want to activate, you need to target the right spots on the spinal cord,” he says. “The leads that have been originally designed for pain don’t provide the ability to do this.” The stimulators can also be programmed to deliver signals timed to the pattern of locomotion.

Hofstöetter agrees that, compared with versions meant for pain relief, this is a “more sophisticated, high-end technological” implant. But, she notes, no study “has specifically investigated whether such level of technological complexity would be” better than conventional devices.

No matter the device—there are about a half-dozen companies working on these kinds of spinal stimulators, Oh points out—the effect is not like turning on a switch. As the Nature report describes, for the patients with chronic paralysis to walk, it required five months of physical therapy, with one- to three-hour sessions up to five times a week. Restoration of function may also mean training for specific motions.

“Getting people out of wheelchairs and walking independently is a miracle,” Oh says. In the current study, all nine participants could walk, some independently and others with assistance. But this stimulation is not a panacea for paralysis, he notes: It cannot repair the ability to run, dance, or kick.

Not every participant responds to stimulation in the same way. And, taking a broad view of spinal cord stimulators, the surgically implanted devices used for long-term pain relief have occasionally harmed patients. An Associated Press investigation found that 80,000 incidents had been flagged to the Food and Drug Administration from 2008 to 2018.

The future of treating paralysis

As much as neurosurgeons and scientists praise the results that stimulators have shown in experimental settings, for the past decade, access to the implants has been generally limited to participation in small trials. Hofstöetter estimates that, across the planet, fewer than 50 people with spinal cord injuries have electrical stimulators. Yet most people with spinal cord injuries could be helped, she suspects, as long as the neuronal networks in the lower part of the spine, the lumbar region, are intact. More work is needed to bridge the gap between proof-of-concept studies and the thousands of people living with paralysis or motor disorders, she says.

[Related: Why doctors almost never say cancer is cured]

A small clinical trial, called STIMO, is underway, led by researchers in Switzerland; the nine participants from the Nature study are enrolled in that trial. But larger tests would be needed for the FDA or other regulators to permit these devices to be used outside experimental settings. “There needs to be a pivotal clinical trial—that’s the first step that needs to happen,” Squair says. 

Some patients with paralysis also say they would prefer stimulation to address functions other than walking, such as control over their bowels, arms, or sexual organs. Ongoing research includes stimulation that targets the bladder, and other studies involve the upper spine in an attempt to improve hand function. To walk again may be just the start, but before that, more mysteries remain.  The exact way this motor function is restored continues to be a puzzle. Thanks to this neuron study, Oh says, we have a target—but we still don’t know the mechanism.

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The insect world teems with beautiful and dazzling species. Silverfish are not one of them. The insects resemble tiny fishing sinkers, with their teardrop proportions and lead-gray complexions. They dwell in basements and musty rooms, where they nibble on dandruff and book bindings. And unlike most insects, they don’t have wings, instead scuttling through life on their bellies.

But these household scavengers are glimpses of history before mammals or dinosaurs—peeks at the mysterious first insect, which paleontologists believe looked something like a silverfish. That primordial creature couldn’t fly, either. Flight transformed insects, launching them on a trajectory to abundance: Today, across nearly every corner of Earth, some 10 quintillion insects hunt, pollinate, and digest. When insects first existed, though, such vastness wasn’t the case. Wings—and winglessness—can explain why. 

Preserved insects are patchily found across time and space, with fewer than 10,000 species described in the scientific literature. Although fossil ants alone outnumber fossil dinosaurs, the insects tucked in rock and amber are a fraction of the estimated 5.5 million living species. “The trend that we see across the fossil record is that insects are really sparse,” says Jessica Ware, a curator at New York’s American Museum of Natural History, who studies how dragonflies evolved. 

[Related: A close look at amber fossils that have stuck through the ages]

Complicating matters still is that the oldest possible insect fossils are squashed and small, inviting interpretation but no clear answers. Ware says it’s not unusual for separate members of her lab to come to different conclusions about the same body part; what might look like a fossilized insect wing under a microscope to Ware could appear as a leg to another researcher, she says.

The first proposed insects can be traced to the Devonian, the geologic period that began 420 million years ago. In 2004, a pair of entomologists argued that a pair of 400-million-year-old jaws just one-tenth of a millimeter long must have belonged to the oldest known insect. They also claimed the mandible so closely resembled a mayfly’s, that this insect must have had wings, too. But more than a decade later, another entomologist duo re-analyzed the jaws and countered that the specimen doesn’t have insect attributes—it was probably another leggy, grounded invertebrate, a centipede, they said.

The status of this animal has yet to be conclusively resolved. Paleo-entomologist and University of Hawaii research fellow Sandra R. Schachat thinks the idea of a 400-million-year-old flying insect is difficult to support without any fossil wings to point at. A fragment of compound eye uncovered on the other side of the Atlantic is more clearly insect-like, she says.

The remnant was found in Gilboa, New York, amid the fossilized tree trunks of the world’s first known forest. The eyeball, several million years younger than the Scottish jaw, “really, really looks like it should belong to Archaeognatha,” Schachat says, referring to the order biologists use to classify bristletails, wingless relatives of silverfish who are still around today. 

But for tens of millions of years after that eyeball, there’s zilch in the fossil record for insects. Schachat and other entomologists call this the “Hexapod Gap,” which refers to the six legs—“hexa” plus “pod”—of insects and their close cousins. This gap lasted from around 385 million to 325 million years ago, until, on its more recent side, buggy parts are once again sprinkled through the fossil record.

In a paper published in 2018 in the journal Proceedings of the Royal Society B, Schachat and her colleagues examined several theories why ancient insects had a vanishing phase. Perhaps something about the environment changed, and only a few insects survived in that 60-million-year-long blank space. Or maybe the animals weren’t actually missing—instead, only their fossils were. 

In the end, the team found that environmental causes didn’t seem to explain the missing bugs. They estimated amounts of oxygen in the Devonian air from preserved chemical traces, and determined there should have been a sufficiently healthy atmosphere for ancient insects to breathe. 

What’s more, that era contains fossils from spider-, centipede-, and millipede-like animals, all about the size of insects and made of similar stuff. “We see way more fragments of arachnids and of centipedes than we do of possible insects,” Schachat says. That’s a sign the right kinds of sediment existed to preserve little invertebrates.

Instead, Schachat and her colleagues say this gap reflects that insects were a lot rarer back then. What made the difference in allowing bugs to take over the modern world, they concluded, was wings—specifically, that the Hexapod Gap reflects a period when insects hadn’t yet evolved them. “The fossil record may accurately record the transformative impact of the evolution of insect flight,” they wrote in the study.

Before wings, insects were confined to crawl, like silverfish still do, or were otherwise reliant on the prehistoric equivalent of hitching a ride in luggage. Then insect wings developed—the first fossil evidence of them dates to 324 million years ago, just after the Hexapod Gap ended. And, suddenly, the sky was theirs to inhabit. 

Aside from a few hundred known species of bristletails, silverfish, and their relatives the firebrats, almost all insects have wings—or, in the case of groups like fleas, lost those appendages from flying ancestors in their evolutionary history. “Once winged insects do appear in the fossil record, all of a sudden they are the vast majority of what we see,” Schachat says.

Sporting wings spiced up how insects looked and behaved—changing how they caught food, how they mated, and how they evaded predators. Flying insects could be bumblers or darters, small creatures or large ones. Some species grew to gigantic proportions: 300-million-year-old insects called griffinflies had wingspans that stretched over two feet.

“Having wings allows you to open your niche space,” Ware says. “There was the entirety of the atmosphere that wasn’t being used.” 

Insects beat bats, birds, and pterosaurs to the air by hundreds of millions of years—and thrived there. “There’s very, very good reason to believe that wings facilitated the diversity of insects and the abundance of insects,” Schachat says. They branched out into new species, taking to not only the skies, but also burrowing into the soil and swimming through fresh water as they traveled into new areas and claimed novel ecological roles. By inhabiting so many corners of the planet, insects shaped life.

[Related: The land of lost fireflies is probably a humble New Jersey bog]

For such influential organisms, insects remain brimming with secrets. How they learned to fly in the first place is an open question. There are no discoveries yet of an ancient insect with intermediate wing-like structures, Schachat says. Maybe an early species experimented with soaring; modern arboreal bristletails have been observed gliding toward tree trunks. Perhaps, according to another hypothesis, insect wings began as gills.

Tracing insects to their roots isn’t simply an academic exercise, Ware says—it’s essential. These animals are so key to agriculture and human diets that, if every insect were snapped out of existence, our species would die off in about three months, she points out. 

“Understanding their evolution is understating 400 million years of life on Earth,” Ware explains. “It’s the closest thing we’re going to get to a time machine.” We live among both extremes of insect evolution—the animals that buzz and flutter above us, and the primitive silverfish at our feet, heirs to an earthbound lineage. 

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