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It was an idyllic late-summer morning in the Alps—August 23, 2017, a Wednesday—and Anna Giacometti, mayor of the , had just opened her office windows so she could enjoy the mountain air. She was sitting at her desk, chatting with a constituent, when suddenly, a noise—deep, loud, and rumbling—made her turn in her chair.

Giacometti walked a few steps to her window and looked toward the sound. It seemed to echo down the dark, narrow Bondasca Valley, which opens onto the small plain where Bondo sits. She knew that at the top of this valley, just a few miles away, the summit of Piz Cengalo loomed 8,000 feet above her village. She also knew, because geologists had warned her, that an entire flank of that craggy, steep-sided mountain was on the brink of collapse.

On a geologic timescale, mountains are always moving. The rolling Appalachians used to stand as high and jagged as the Andes, while the Himalayas continue to grow by about half an inch every year. But dramatic shifts in the earth’s climate are forcing very rapid and intense changes in today’s mountain landscapes, environments that are keenly sensitive to any fluctuation in heat or rainfall. Since the start of the 20th century, the average temperature in the Alps has risen by 3.6 degrees Fahrenheit—more than twice the global rate. Exposed to such heat, prehistoric ice is melting and ancient glaciers are pulling away from the steep mountain flanks they’ve buttressed for millennia. The earthly chaos that results from these changes represents a genuine threat to anyone who spends time in alpine regions. And the risks, scientists say, are growing.

In Bondo, Giacometti could hear the rumbling, which wasn’t dying down. Standing at the window of her third-floor office, she looked at the early warning beacon that had been installed along the banks of a creek that snaked down from the valley above. Normally, the system’s lights were off. But on that clear August morning, for the first time, they were red. She knew what that meant: a large, moving mass had snapped a trigger line high in the valley above. 

The mayor turned and ran out of her office, shouting to her coworkers as she sprinted down the stairs and out the front door of Bondo’s small town hall. She stood in the parking lot, searching the forested slopes overhead for any clues as to what was going on. An immense cloud of dust was rising above the treetops that lined the horizon. The rumbling continued.

Something was coming down, that much was certain. She didn’t know what it would look like or when it would arrive. But she knew that people in and around Bondo had to get out of its way, and they didn’t have much time.

Giacometti started to run. She and a colleague went from house to house, knocking on doors and shouting that everyone had to evacuate. At 9:30 A.M., about 150 people were inside the homes, offices, and shops that line Bondo’s ancient cobblestone streets. Some were scared when they heard her shouts, while others doubted there was any real threat. Ultimately, all of them heeded her appeals to leave and took shelter with friends or family in nearby villages.

And then it came: a fast-moving river of thick, lava-like mud poured out of the valley and into the village. Over the next several hours, the mud and the blocks of granite that it carried wiped out an old stone bridge, obliterated barns and houses, and eventually overflowed the catchment basin that had been constructed to mitigate just such a catastrophe. Giacometti, who was born in Bondo in 1961, watched helplessly from higher ground as the muddy wound cut deeper and wider through the middle of the village. Everyone was in shock. It felt like they were watching a grim scene in a movie, a reality that no one could comprehend.

By the end of the day, the river of mud had slowed and authorities were beginning to grasp the extent of the destruction. The roar of churning rock and mud had been replaced by the thrum of helicopters circling overhead, their occupants searching for signs of life among the rubble. No one from Bondo had been killed or injured, but the news had reached town that, high above them, eight people were missing.

The mood over breakfast had been cheerful in the , a century-old stone mountain hut perched on the edge of the Bondasca Valley, 4,000 feet above Bondo and 4,000 feet below the bare, towering summits of Piz Cengalo, Piz Badile, and Pizzi Gemelli, peaks the celebrated Italian climber Walter Bonatti once described as “some of the greatest granite colossi in the Alps.”

The Rifugio Sciora can accommodate up to 40 people, but the busy summer season was nearing its end; only nine guests had slept in the hut the night before. The hikers and climbers had come alone or in groups of two, but they all sat together for breakfast on that bright, sunny morning, chatting easily in German, their common language, as they sipped coffee and tea and enjoyed fresh bread with butter and jam or thin slices of ham and cheese.

At 8 A.M., just after breakfast, eight of the nine guests shouldered their packs and said goodbye to Reto and Barbara Salis, the husband-and-wife team who were managing the hut that season, their ninth summer in a row. Before the hikers left, Barbara Salis told them they’d have to come back someday. Everyone agreed.

The hikers set off together, continuing the conversations they had started over breakfast as they made their way along the trail leading down toward the Bondasca Valley. The sun was warm, the sky was clear, and they were all on their way home—back to the jobs and bills and chores that waited for them in their day-to-day lives in Germany, Austria, and other parts of Switzerland.

As they neared the edge of the valley, the eight hikers passed a sign—more than six feet high, written in German, Italian, and English—alerting them that the area they were about to enter was closed because of a high risk of rockfall. An arrow pointing to the trail on which they stood was marked with a warning, spelled out in type that was bold, red, and all caps: “LIFE-THREATENING!”

At 9:30 A.M., Barbara was in the kitchen, preparing food for that evening’s guests, when her husband, who had stepped outside, started to shout. “Barbara! Barbara! Something’s coming!” he said. “Look!”

She ran outside, stopped, and screamed. They had seen landslides tumbling off the Piz Cengalo before. Ever since the first big one had come down in 2011, the mountain had always been moving. But there had never been anything like this.

A wall of rock as tall as a 25-story building—an entire flank of the 11,053-foot Piz Cengalo—was collapsing. It was as if the granite sides of the peak had turned to sand, so fluidly was the mountain falling into itself and cascading down. Billowing clouds of pulverized rock surged up from the valley floor, enveloping the forested slopes in a thick, gray darkness that nearly reached the spot, high on the ridge, where Reto and Barbara now stood.

Immediately, Barbara thought of the eight hikers who had left the hut just an hour and a half earlier. She ran inside to find where she had recorded their names and cellphone numbers. She dialed each of the numbers again and again and again. None of the calls went through.

The landslide that inundated Bondo was probably centuries, if not millennia, in the making. Scientists who have studied the catastrophe say that understanding what happened that day can help us prepare for the increasingly volatile future that climate change will inflict on mountains and the people who live in them.

The first major rockfall that scientists recorded on Piz Cengalo happened in 2011, two days after Christmas, in the darkness of early evening. During that episode, the debris didn’t travel as far as Bondo, so no buildings or roads were damaged. But the event didn’t go unnoticed. What caught the attention of Florian Amann, a geologist at the University of Aachen in Germany and a specialist in the stability of rock slopes, were the chunks of blue ice that he saw wedged in the rocky scar of the landslide in photos that were taken soon after the event.

“If you find such blue ice in the detachment zone, you know there’s permafrost,” Amann later explained. “We wanted to know what is the role of this ice in destabilizing such big granitic rock walls in the Alps.”

Amann and his colleagues traveled to Bondo the following summer and set up radar equipment to monitor Piz Cengalo’s crumbling flank from a safe distance; rockfall made it too dangerous for them to get close. They could see that a large section of the mountain was moving, but for several years it did so at a fairly slow and predictable pace. Then came the summer of 2017. When the geologists sat down near the end of July to analyze their latest data, they saw that the speed of the mountain’s movement had tripled from the year before. 

“We were completely shocked,” Amann says. “It was not only a little piece of rock which was moving. It was the entire face of the Cengalo.” 

At that point, they knew that something big was going to come down, and soon. Amann and his colleagues contacted the authorities in Bregaglia, Bondo’s district, to alert them to the mountain’s increasingly fragile state. The scientists worked with local officials to create warning signs that described, with remarkable precision, the size and extent of the landslide they expected in the very near future. The signs—the same panels the eight hikers would walk past on the morning of August 23—also warned people that by entering those areas, they were putting their lives at risk. Local authorities posted the signs in Bondo and along hiking trails throughout the area. After that, all they could do was watch and wait. 

Did changes to ancient ice weaken the side of Piz Cengalo? It’s possible, but because the mountain was too dangerous for the geologists to monitor closely, we’ll probably never know for sure. Amann and other scientists who have studied the area agree that while climate change could have hastened things along, deep joints in its structure were weakening to such a degree that collapse, at some point, was inevitable. 

After the event, one question left scientists scratching their heads. How did a dry rock avalanche, coming at the end of an unusually dry summer, trigger such a wet, voluminous, and destructive flow of mud? The answer was apparent not in the rockfall itself, but in the path the debris took down to the village.

The glacier that used to fill the valley below Piz Cengalo retreated long ago, but on the eve of the 2017 landslide, a relatively small and unnamed fragment of ancient ice still lay hidden in a dark corner of the Bondasca Valley. When the side of Piz Cengalo collapsed, more than 3 million cubic meters of rock—enough to fill the barrels of nearly 400,000 concrete mixing trucks—landed on top of this small glacier, instantly transforming nearly all of its ice into water. The water shot out, mixing with rock and other debris as it surged down the steep valley, slamming onto water-laden sediment that previous landslides had deposited along the valley floor. The pressure of the falling debris compressed the sodden material like a fist squeezing a soaked sponge, triggering the explosive release of even more water into the valley and down toward Bondo. 

Such sequences of cascading events, which geologists call , are incredibly difficult to predict. They’re also likely to become more common in the years ahead because of climate change. More intense rainfall is part of the problem, as is the thawing of permafrost—that ancient, frozen glue that holds together the summits of many of Europe’s highest peaks. 

But in the Alps, perhaps nothing could be quite so destructive as the melting of the range’s ancient rivers of ice. More than half of Switzerland’s very small glaciers will probably disappear completely within the next 25 years. By the end of this century, more than 90 percent of the volume of glaciers across the Alps could vanish. That means more risk, says Matthias Huss, a glaciologist at ETH Zurich and the head of Switzerland’s glacial monitoring network.

“If these very small glaciers are gone, and we have relatively steep mountain flanks with a lot of debris lying around, the danger of mudflows is increased,” he says, noting that the potential for rockfall will also rise, posing threats to hikers and mountaineers. The catastrophe in Bondo can be instructive, he added, for scientists and local authorities alike. 

“There is probably no other event of this sort globally where such a lot of data exist and where scientists can really go into detail to understand what happened,” Huss says. “This chain of processes—from the rockfall to the glacial erosion to the mudflow and so on—can certainly be transferred to other potentially dangerous sites in the Alps.” 

Risk will also increase in other mountain ranges as retreating glaciers, melting permafrost, and intense rainfall take a toll on high, glaciated peaks from the Himalayas to the Andes to the high country of Alaska. (In 2015, the collapse of a mountain slope near the bottom of Alaska’s retreating Tyndall Glacier that reached a height of roughly 650 feet.) Few of these other areas are as densely populated as the Alps, but many lack even modest protections for the people who inhabit them: 156 people were buried a mountain village in Nepal in August 2014. 

One clear lesson from Bondo’s catastrophe is the importance of keeping a close watch on the slopes overhead. Even in Switzerland—a country famous for its miles-long mountain tunnels and intensive barriers against snow avalanches—no feat of human engineering could ever stop a flow as massive as the one that came down the Bondasca Valley. The best anyone can do is watch and prepare for what could come next.

The early warning system that was installed in Bondo in 2013—when it was already clear that the mountain was unstable—likely saved lives in the village. Geologists were closely monitoring the area, and when the time came, authorities were quick to heed the scientists’ advice. Such a set of circumstances is not uncommon in Switzerland, a wealthy country that has for centuries actively managed the natural hazards that threaten many of its communities. In other mountainous parts of the world—where villages may be even more remote and where government pockets may not be so deep—the scope for destruction could be much greater.

The road to Bondo is winding and dark. As I drive into the village one morning in early summer, nearly two years after the landslide, I’m struck by the sense, at once comforting and claustrophobic, that the valley is closing in around me. 

Anna Giacometti still occupies her office on the third floor of the town hall, but she likes to get out when she can. After showing me the view from her window, she walks with me around the village. Nearly 100 buildings were damaged in the landslide, and about a third of those were destroyed. Rebuilding work began immediately, with Swiss soldiers and government crews working late into the night to speed the reconstruction. The first group of families was allowed to return home on October 14, less than two months after the landslide; nearly all had decided to return. Giacometti shows me the cobblestone square where they drank wine and ate roasted chestnuts to celebrate the homecoming.

The rebuilding work cost nearly $42 million, most of which came from the Swiss government and the local canton of Graubünden. But Bondo—which made headlines across Europe and beyond for a few days after its catastrophe—also received nearly $15 million in donations from around the world. With that money, the community built a new playground, as well as a suspension bridge that traverses the scar cut by the debris flow, like a stitch on a wound.

There’s more work to be done—another $25 million worth of reconstruction and fortification is being planned for this year and next—but life in Bondo seems to have resumed its normal pace. And while plenty of hikers, mountain bikers, and rock climbers continue to explore the area’s trails, the Bondasca Valley remains closed. Geologists have detected movement in another 3 million cubic meters of rock on the flank of Piz Cengalo, roughly equal to the volume of the 2017 landslide. Giacometti says they’re certain that the mass of material is going to come down—the main question being whether it will peel off in a series of smaller events or in a single, devastating blow. Bondo now has a new and more sophisticated warning system that continuously monitors the face of Piz Cengalo to detect even the slightest shifts in the rock. It also dug a catchment basin to prepare for the arrival of another flow. The village is as ready as it can be.

Giacometti leads me around the back of the church and into the cemetery. A large granite stone that was salvaged from the debris has been set down in a quiet corner in the shade of a yew tree. A plaque bolted to the stone commemorates the eight people who were lost that day—no trace of them has ever been found, and their names and ages have not been made public. Someone has left a large bouquet of roses that can’t be more than a day or two old. A white ribbon winds around a wicker basket overflowing with greenery; on it, someone has written, “Wir vergessen euch nie.” We will never forget you. 

“The families come here often,” Giacometti says, kneeling. “I think it’s important for them to have somewhere to go.”

We continue through the village and bump into a kind-looking man with a trim white beard who, Giacometti later tells me, was deeply distressed in the aftermath of the landslide. But he keeps his composure as he describes how his house was overrun with mud and rock and how he and his wife decided not to leave the valley but to stay and rebuild.

He still feels scared sometimes, he tells me, his eyes turning toward the dark entrance to the Bondasca Valley—especially in summer when a strong storm comes. But he’s quick to add that he never thought of leaving. Here in the mountains, he says, this is just part of life.