The final minutes before floodwater crashed through Nepal-China border

A dark wall appears from behind the buildings at Gyirong, on the Chinese side of the Nepal-Tibet border. At first, it looks almost like a black cloud. Then it gathers shape and speed: water, mud, boulders, ice and rock surging down the valley. The CCTV footage, whose timestamp reads 10:59:53, went viral.

For many, it was the first visual evidence of the sheer scale of the destruction that was unfolding.

What happened in the minutes that followed is still difficult to reconstruct. But fragments of video and satellite imagery show much of the border complex swept away, with buildings that once crowded the valley floor simply gone.

Electronic immigration records from Nepal's Rasuwagadhi border post are filling some gaps, as well as insights from its only surviving staff member. Tika Ram Pokharel, who happened to be in Kathmandu sitting an exam on the day, tells the BBC the crossing was unusually busy when floodwater hit.

"This is a historic travel route between Kathmandu and Tibet. The area is geologically vulnerable, characterised by deep gorges and steep rivers that flow at high velocity, carry heavy sediment and can bring down huge boulders," says Ajay Dixit, a Kathmandu-based hydrologist.

It is a place where pilgrims heading towards Mount Kailash cross paths with traders travelling between Nepal and China, truck drivers, customs officials, tourists and people who live along the border.

On the morning of the flood, more than 100 Indians may have been in or around the crossing. At least 275 Indians remain missing in Nepal, according to India's foreign ministry.

Nepal's immigration department has been trying to reconstruct who passed through Rasuwagadhi in the final hours before the disaster.