The deadly avalanche on Everest earlier this month wasn't technically an avalanche. It was an "ice release"—a collapse of a glacial mass known as a serac. Rather than getting swept up by a rush of powdery snow across a slope, the victims fell under the blunt force of house-sized ice blocks tumbling through the Khumbu Icefall, an unavoidable obstacle on the most popular route up Everest. The worst accident in the mountain's history has effectively ended the 2014 climbing season. And some see global warming as the key culprit.