Matti Masing remembers the day his mother first dragged him into the cave. It was 1966, and he was 13 years old. “Just a schoolboy,” he says. At the time, he was nervous to enter the Piusa Sand Caves, a complex underground maze with high ceilings and twisting passageways built by sand-glass miners in southern Estonia more than 40 years prior. But his mother, Linda Poots, needed to; she was a bat researcher, and she had a hunch.

“I was quite scared,” he remembers. “It was so dark in there.” Today, the caves are ornamented with electric lights and safety railings. But back then, they’d been recently abandoned. To Masing, they felt musty and feral.

Mother and son walked the sandy floors with candles in hand. The circles of light led the way, catching against whorls and ridges in the hand-carved walls. Vaulted ceilings soared up to 40 feet overhead, and shadows crowded the arched passageways. Masing couldn’t help but hold his breath. The labyrinth looked like something from a fantasy world—not unlike the Mines of Moria from Lord of the Rings, or Aladdin’s treasure cave among the dunes.

Piusa Sand Caves
First dug in 1922, the Piusa Sand Caves extend more than 12 miles into the earth—and provide habitat for about 4,500 hibernating bats. Courtesy of Piusa Sand Caves

Then, finally, the candlelight illuminated a single point above them. They found what they were looking for: a ceiling covered in the furred humps of dozens of bats. Poots smiled. This was it, a small piece of the Baltics’ biggest bat colony—and ground zero for what would become a stunning recovery of bat populations across the European continent.

Bats at the brink

Estonia is a small nation tucked between Finland, Latvia and Russia, against a bend in the Baltic Sea. Temperatures remain around freezing about six months out of the year. Hills are scarce—the country’s highest point is just 1,000 feet above sea level—and caves are even scarcer.

“All the caves in Estonia are man-made,” says Karolin Tuul, a guide at the Piusa Sand Caves. “There just aren’t very many here.”

But bats abound. The nation is home to 12 species, seven of which hibernate domestically—as opposed to migrating elsewhere. And for that, they need a hibernaculum, a place where they can sleep totally uninterrupted for six months at a stretch. They need their peace because bats don’t hunt during hibernation; instead, they rely on built-up fat and energy stores. If foot traffic or industrial noise wakes a bat, it shifts. That in turn creates a domino effect, waking all the other bats and making the whole colony waste precious energy. Too many interruptions, and the bats won’t make it through the winter.

For bats, good sleep hygiene is a matter of life and death. But on a continent as packed with people as Europe is, that kind of perfect solitude and personal space is hard to come by.

Scientists aren’t entirely sure where bats went to hibernate before artificial caves started appearing in the 1900s, says Oliver Kalda, a bat researcher with the University of Tartu. (He and his brother Rauno Kalda have been studying Estonia’s bats for more than a decade.) Some species may have found rock crevices to hibernate in. Others took advantage of sheep and cow sheds, where stamping livestock and decomposing manure provided a constant source of heat.

Much of this is conjecture, says Rauno Kalda. But what we do know is that, starting in the 1940s, deforestation, commercial agriculture and intensive development crept across Europe, fracturing some habitats and decimating others. Populations across the continent plummeted. By the 1950s, bat populations had entered a steep decline. For a while, it seemed like many species would disappear altogether.

Poots was determined not to let that happen. And she felt Piusa might be the key.

The Piusa Sand Caves

The 1920s brought a rush of economic prosperity to Europe, along with a flurry of postwar reconstruction. Glass was in high demand. But without modern equipment, the sand required to make that glass remained hard to come by. Then miners discovered the deposits at Piusa.

Piusa, Estonia, is home to some of the most accessible fine sand in the Baltic states. Soft sandstone buffs rise from the forest. In some areas, you can scoop sand straight from the forest floor. In 1922, miners began excavating some of the richest pockets, carving long tunnels into the earth. Today, Piusa’s sand caves are one of the architectural wonders of southern Estonia. They sprawl through the earth, a 12-mile labyrinth twisting beneath the forest.

“They’re so huge—it’s hard to believe how big they are,” says Tuul. Most of the caves are pitch-black. But in one of the caverns, “you can see the sunlight coming through the roof and the different layers of sandstone as you walk through.” They’re beautiful, Tuul says. They were also absurdly productive.

For 44 years, the caves were one of the most important sources of fine sand used in glassmaking throughout the Baltics, Russia, Poland, Germany and beyond.

“The sand was quite valuable,” Tuul says—which is why miners tolerated the time-intensive toil of carving walls in the dim light of oil lamps, and took care designing the columns and vaulted ceilings to hold the weight of the ground above.

The mines remained important until the middle of the 20th century, when modern techniques made it more efficient to strip-mine the nearby forest than to keep etching tunnels into the earth. Around 1966, the caves were abandoned altogether.

When miners first began pulling out of the caves in the 1930s and ’40s, bats quickly filled the void. At first, they used the same entrances the miners did. Later, small ceiling collapses provided alternative exits.

Once bats find a suitable hibernaculum, they tend to return year after year, which attracts more bats. Over time, Piusa’s population snowballed.

Matti Masing
Matti Masing stands by the entrance to a cellar during some early fieldwork in 1979. Inara Buša

Masing says that his mother, who had trained under leading Soviet zoologists at the University of Moscow, was among the first to discover bats living in the caves in 1948. Over the next few years, Poots set up a series of experiments to study bat longevity, behavioral patterns and population size.

Back then, modern research equipment wasn’t always easy to come by. Sometimes Poots explored caves with a dim hand torch. Other times, just a naked bulb or a candle.

“It wasn’t easy to obtain maps back then,” says Oliver, later adding, “So, in the old days, researchers had hand-written or hand-drawn maps that they passed from one to another.”

Research methods could also be somewhat rudimentary. Poots and other scientists would often bring armies of students into the caves. They would spread out, industriously plucking bats from the walls with long sticks and banding every single one of them. Oliver points out that this practice has since been found disruptive to bats and has fallen out of favor.

Still, Poot’s research was considered the best practice for its time. In the 1950s, she began publishing her findings. Word quickly got out, and over the next few decades, scientists flocked to the caves from Finland, Latvia, Norway, the Netherlands and beyond. By the 1970s, this growing international group of scientists had completed a full survey of the cave populations and had both good news and bad news. The good: The cave was perfect habitat for the more than 4,000 bats that filled it each winter. The bad: If it wasn’t protected, these bats would have nowhere to go.

Saving Piusa

Poots and Masing realized they had to do something. So, in 1980, they started writing letters.

“My mother and I together made a presentation to show the government,” Masing says. “Our report said that we had found so many bats in the Piusa Caves and that it was very important to protect it.” Within a year, the government of Estonia—then the Estonian Soviet Socialist Republic—passed legislation protecting the caves and prohibiting entry during hibernation season.

Of course, that didn’t solve the whole problem. Yes, the Piusa Caves provided a refuge for bats within a 125-mile radius, according to Tuul—but even that wasn’t nearly big enough to serve all the bats in the region. The protections needed to be more comprehensive.

Matti Masing and Linda Poots
Masing first began studying bats with his mother, Linda Poots, in the late 1940s. Courtesy of Matti Masing

At the time, Masing had begun researching another understudied hibernaculum: root cellars.

His interest in cellars had started by accident in 1975. He was working as a field biologist banding birds and, like many young solo field biologists, found himself with way too much time on his hands.

“One day, I was exploring in the forest, and I found the ruins of an old manor from a German landlord,” he says. German landowners once built manors all across Estonia. When Estonia first got its independence roughly 100 years ago, the German landlords went back to Germany, and many of the manors were destroyed, Masing says. “But the cellars are still there.” Many of these cellars are carved into soil or limestone. Some are up to 160 feet in length, Masing says.

Masing looked at the ruins. He had a flashlight with him, so he decided to go poking around. Soon his flashlight beam flickered over a sight he’d been trained as a child to recognize: a cluster of hibernating bats. No, not just a cluster—hundreds of them.

Masing watched them, just as he did that day in the Piusa Caves so long ago. He realized maybe bats were his calling, too.

By the 1980s, Masing had built up enough evidence to show that root cellars and ruins were a significant source of bat habitat. Conservationists took his work and ran with it. Bat advocacy groups began following Masing and Poots’ precedent, fighting to protect certain underground sites the same way they had fought to protect Piusa. Thanks to that advocacy, officials ultimately passed regulations prohibiting tourists from visiting the tunnels around Tallinn, the nation’s capital, during bats’ hibernation season. They later expanded those same kinds of regulations to other caves and fortresses near the city.

“Piusa was one of the first places where they put up this kind of a system,” says Rauno. For that reason, he adds, “The significance of Piusa for bats is quite noteworthy.”

Kick-starting a movement

Similar efforts spread across Northern and Eastern Europe. In Poland, Lithuania and Latvia, researchers found bats in old mines, abandoned farm buildings, crumbling fortresses—and Nazi bunkers. Sometimes, that gave locals pause. Many consider the bunkers reminders of a dark past. Some wanted them sealed up or destroyed. But bat advocates fought to keep them open—and then retrofitted them for bats’ use. In Poland and Lithuania, more than 40 World War II shelters have now been renovated to include bat boxes and roosting structures, giving them a new life.

Bats in Kaunas Fort
Bats nestle in a crevice in Estonia’s Kaunas Fort, which Masing studied during a research trip in 1979. Matti Masing

By the early 2000s, bat populations had begun to bounce back. According to a report published in 2014 by the European Environment Agency, bat populations had increased by up to 43 percent between 1993 and 2011.

Today, Estonia’s bat populations are mostly stable, with a few species still increasing, say the Kalda brothers. (They add that the populations of a few other species may still be in slight decline.) Biodiversity experts with the Estonian Environmental Board have also removed two of Estonia's most prominent species from the “vulnerable” category and instead marked them as species of least concern. However, new threats are on the rise. Wind turbines—which are being installed across Estonia’s forest landscapes to help the country hit a renewable energy target by 2030—are now a leading concern for bat conservationists. The spinning blades have the potential to kill bats if they’re not thoughtfully located. Much of the Kalda brothers’ work today involves consulting with energy companies to help them identify bat-safe turbine sites. Fortunately, Oliver says, some clever solutions are out there.

“In some countries, like Germany, turbines are curtailed when the bats are flying,” he says. From the beginning of May to the middle of September, when bats are active, algorithms cut turbine rotations whenever the conditions are good for flying. So, on a calm, dark night, turbines shut off. When it’s too rainy for bats to hunt, the turbines continue at full speed.

As for Piusa? Thanks in large part to Poots’ work, the cave is still home to the largest bat colony in the Baltic states and is still considered one of the most important sites for bat conservation in the region. The deeper caves are now roped off to the public, and the above-ground visitor center—a building shaped like a bat—is largely dedicated to bat education. Each summer, when the flying mammals are gone, more than 10,000 tourists come to see the hand-carved tunnels, says Tuul. And each winter, the tourists trickle out, making room for the real stars of these caves: the bats.

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