A spectacular view of Mount Everest was not what most stunned Hokkaido University geoscientist Evgeny A. Podolskiy during his first trip to the Himalayas in October 2017. What shocked him while working and living in the area were the loud, reverberating booms every night.

“The ice was cracking up,” says Podolskiy, who has done research in several other glacier environments around the world, including Greenland and the Alps. “I've never come across anything like this before.” Aside from one anecdotal observation made in the Arctic, there was no scientific record of such glacial fracturing at night.

The cracking is bad news for the more than a billion people in Asia who rely on these icy reservoirs for water. “This kind of wear and tear on a daily basis can make glaciers more fragile and therefore melt more easily,” meaning there will be less water available over time, Podolskiy says.

To home in on the source of the cracking, Podolskiy and his colleagues installed seismometers throughout the Trakarding-Trambau Glacier System in eastern Nepal—the first such attempt in the Himalayas. The team noticed an interesting pattern: the seismic rumbles came from ice surfaces free of debris. And larger drops in nighttime air temperatures resulted in stronger seismic signals, the team reported last September in Geophysical Research Letters.

In contrast, ice blanketed with a layer of loose rocks made little noise, and it was totally silent if the rubble above the ice was thicker than two feet. “The debris, in effect, protects glaciers from temperature oscillations that make the ice expand and contract cyclically,” Podolskiy says. “When the temperature drops sharply, as it does at high elevations, the rapid contraction of unprotected ice breaks it up.”

That protection is limited, however, because less than one fifth of the glacier surface in the Himalayas is covered with debris.

Walter Immerzeel, a glacier hydrologist at Utrecht University in the Netherlands, who has worked in the Himalayas for more than 16 years and was not involved in the study, called the finding fascinating. The study “points to a new way that the stability of glaciers can be threatened,” Immerzeel says. Cracks not only cause mechanical damage; they also act as conduits for water and heat and can greatly accelerate ice loss, he adds.

As Podolskiy's team plans its future studies in the Himalayas, “a pressing issue is how cracks develop and evolve throughout the year and how this affects water flow within the ice,” he says. “This is crucial for a better understanding of the future of Asia's water tower in a world of changing climate.”