NEW DELHI (AP) — On Everest, everybody knows that the Khumbu Icefall is dangerous. They’ve known it for generations, since the first great Everest mountaineer, George Mallory, turned away from the Icefall in 1921, insisting it was impossible to pass.

It is a river of ice, a kilometer (half mile) or so of constantly shifting glacier punctuated by deep crevasses and overhanging immensities of ice that can be as large as 10-story buildings. It can move two meters (six feet) in just one day. Crossing it can take 12 hours.