More than 3,000 people were missing after floodwaters and mud crashed through Himalayan river valleys in Nepal and the Tibet region of China.

A huge torrent of water, mud, rocks and ice swept away settlements on Wednesday, killing at least 781 people in Nepal and 16 in Tibet, according to the authorities. Over 2,500 people were missing in Nepal, the country’s disaster management authority said. In Gyirong County, in Tibet, 546 people remained missing, Chinese state media reported.

Shisir Khanal, foreign minister of Nepal, said on Thursday that the floods were “one of the largest natural disasters we have experienced in recent memory.”

“Towns after towns have been completely wiped out in a span of a couple hours,” he added.

Where did the floods hit?

The torrents rushed through the valley of the Trishuli River, which is flanked by steep mountains and straddles the border between Tibet and the Rasuwa district of Nepal.

Images from the disaster captured the terrifying speed and force of the floods. Surveillance footage verified by The New York Times showed crowds running from the surging water before torrents engulfed buildings and vehicles at the Gyirong border crossing.

The Chinese state news agency Xinhua reported “significant” casualties at Gyirong, and other footage released by Chinese state media showed surging waters and mud overwhelming buildings, uprooting trees and rendering roads impassable.

Video and images from Nepal showed the debris-choked river wiping out bridges, and towns partially buried in mud. Tens of miles of roads and more than 40 bridges were destroyed, and several hydropower projects sustained heavy damage, according to Nepali officials.

How can a flash flood happen without heavy rain?

Officials were caught off guard because there was no downpour, according to Narendra Pariyar, the chief district officer of Rasuwa.

Scientists analyzing satellite imagery and seismic data said that a high-altitude landslide coinciding with a glacial collapse most likely set off the flooding. Images showed that a huge section of bedrock beneath a glacier on the north face of Langtang Lirung, a mountain in Nepal near the Tibetan border, had given way at an elevation of about 17,000 feet.

“The rock that the glacier was sitting on collapsed,” said Kristen Cook, a geomorphologist at the Université Grenoble Alpes in France. The combined collapse sent millions of metric tons of rock and ice into the valley below.

The event was so powerful that the U.S. Geological Survey registered it as a magnitude 5.2 seismic event.

As the rock and ice surged down the mountainside, the flow incorporated water and river sediment, becoming a torrent of mud and water that traveled dozens of miles through a network of rivers and tributaries.

Who are the missing tourists?

Hundreds of travelers are among the missing. In Nepal, hundreds of foreign tourists were among the over 2,500 still unaccounted for, the country’s disaster management authority said. The missing tourists included travelers from Australia, Britain, Malaysia and the United States.

Among the missing in Nepal were some 100 Chinese nationals, according to the Chinese Foreign Ministry. In Tibet, about 260 foreign nationals were among the 546 missing, Chinese state media reported. They included 104 Nepalis, 49 Indians, 33 Latvians and 18 Americans.

About 300 Indian nationals were missing in the flood-struck parts of Nepal, India’s foreign ministry said, including many pilgrims traveling in groups to or from Mount Kailash in Tibet. Mount Kailash is a sacred site for multiple faiths, including Hinduism.

Sadhguru, an Indian spiritual guru and entrepreneur whose Isha Foundation arranges pilgrimages to Mount Kailash, said on social media on Thursday that 80 people participating in the organization’s tours had been killed in the flooding in Gyirong.

How common are landslides in the region?

Despite lower-than-average rainfall across the country this monsoon season, local media noted heavy rains in multiple hilly regions that caused deadly landslides and flash floods earlier this month.

In 2024, intense rains caused floods and landslides that killed hundreds in Nepal, including in the densely populated region around Kathmandu, the capital.

Nepal is prone to landslides and floods when heavy monsoons collide with the Himalayas.

The warming climate has made weather events more dangerous and deadly, causing frequent flooding from melting glaciers. Climate change has also intensified rainfall. At the same time, rapid development and haphazard construction have added to the risk that lives will be lost in natural disasters.

Reporting was contributed by Binod Ghimire, Lily Kuo, Mujib Mashal, Suhasini Raj, Hari Kumar, Leo Sands and Pranav Baskar.