Earth is home to 14 “eight-thousanders,” summits that top off at more than 8,000 meters, or 26,247 feet, above sea level. All of these grand mountains tower over the Himalayas, the highest place in the world.
But our planet is dynamic—could there have been additional peaks like these, since lost? “We wanted to know whether, 830 years ago, the Earth and the Himalayas had one more,” says Jérôme Lavé, a geomorphologist at the National Centre for Scientific Research (CNRS) and the University of Lorraine in France.
The answer, according to Lavé and his colleagues, appears to be yes. In a new paper, published in Nature on Wednesday, they’ve found evidence of an ancient landslide that reshaped South Asia’s geography—and linked that to the collapse of a peak that would have once been one of the tallest mountains on Earth.
Lavé says his team first spotted the fingerprints of this medieval landslide not in the Himalayas, but far to the south, near the India-Nepal border, in the flat plains around the Narayani River.
To look for missing mountains, these plains are prime land for geomorphologists—scientists who study the evolution of the land under our feet (or, in this case, the land towering well above everyone but the hardiest mountaineers). Rivers like the Narayani carry sediments downslope, and those sediments can reveal much about the mountains where they originated.
For instance, Lavé and colleagues found medieval sediments with a carbonate content five times higher than average. This mineral fingerprint indicated that something had disrupted the Narayani’s flow. “A giant landslide occurring…seemed to me the most obvious avenue to explore,” Lavé says.
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They began plying uphill to find out more. The Narayani flows through the city of Pokhara, nestled in a valley less than 3,000 feet above sea level. But this is one of the steepest landscapes on Earth: looming over Pokhara is the Annapurna massif, a section of the Himalayas. (The massif’s crown jewel is its tallest peak: also named Annapurna, a proud member of the eight-thousand club.)
By studying images of the Annapurna massif, the team found geographic signs of an old landslide. In one subsection of the massif, called the Sabche cirque, they spotted strange features like pillars and pinnacles, markers of erosion.
The authors needed more samples. Collecting fragments from the plains is one thing. It was another to gather wood and rock from the Sabche cirque—they ventured up into the massif by helicopter. From these parts, they began to build the hazy image of a mountain that existed, long ago, until one catastrophic day around 1190 CE.
“They really managed to capture this event…both at the source as well as at the far sink of these sediments,” says Wolfgang Schwanghart, a geomorphologist at the University of Potsdam in Germany, who was not an author of the paper.
This is what Lavé and colleagues think happened: There once rose a second eight-thousander from the Annapurna massif. Then, it collapsed. The resulting rockslide thoroughly eroded the Himalayan landscape and poured sediment into the valley that now contains Pokhara, from where waters carried it downstream. This event played a major role in eroding the rock, reshaping the massif closer to what we see today.
The paper suggests that large, dramatic landslides may be a significant driver of erosion at high altitudes like this. “This is a mechanism that still needs to be further investigated, but this hypothesis may open new insights,” says Odin Marc, a geomorphologist at CNRS who was also not involved in the research.
What caused the mountain to collapse isn’t clear. A warming medieval climate might have melted mountaintop permafrost that otherwise strengthens the peak. Schwanghart, who has also studied the region’s geology, believes the answer may be earthquakes. He says the chronology indicates that three earthquakes struck Nepal around the time that Lavé and colleagues suggested the mountain collapsed, and one of them may have caused the mountain to topple in the first place.
Whatever happened, the new report reinforces the fact that mountains are constantly changing environments. We might see summits as eternal fixtures on the landscape, but if anything, they are the complete opposite.
After all, Himalayan landslides aren’t consigned to the past. In 2021, an avalanche and rockslide careened down a mountainside in Uttarakhand, India, around 300 miles northwest of Annapurna. The disaster burst a dam, and the resulting flood left some 200 people dead or missing.
If such a rockslide were to happen to Pokhara today, the results could be devastating. Pokhara is Nepal’s second-largest city (after the capital Kathmandu) and home to more than half a million people. Moreover, globally, evidence is mounting that a warming climate exacerbates the risk of mountain landslides. Just last month, the Alpine summit of Fluchthorn, nestled on the Swiss-Austrian border, abruptly collapsed in an event that scientists ascribed to thawing permafrost.
Mountain collapses like these may be more common than we realize. “In Alaska, you would find similar events—but often they go unnoticed, because there is no one around,” says Schwanghart.