Terrifying video shared from Machhakhola in Nepal's Gorkha district shows muddy floodwater roaring down the Budhi Gandaki valley, tumbling massive boulders as if they weighed nothing. The flash flood hit around 4 a.m. local time on October 11, carrying mud, sediment, and debris through a Himalayan corridor popular with trekkers. Authorities said it swept away about a dozen houses, two suspension bridges, and a highway bridge, and it also destroyed a hydropower project camp. At least five people are reported missing, the army says it rescued 49, and local officials said more than 100 foreign trekkers were left stranded.

So what set off the surge? Preliminary accounts reviewed by the International Centre for Integrated Mountain Development suggest a landslide near Chumchet temporarily blocked the river and the blockage then gave way, though the exact cause has not been confirmed. Reports describe it as a dry landslide rather than one triggered by rain, and some glaciologists have not ruled out a glacial or ice-and-rock trigger. The disaster comes less than seven weeks after another flood in Rasuwa, the deadliest natural disaster Nepal has seen since the 2015 earthquake. With steep terrain, loose sediment, and unstable slopes, landslide-blocked rivers can release walls of water and boulders with almost no warning, leaving communities in narrow valleys very little time to escape.