Joe Yelverton was on a rocky outcrop in the Chugach mountains, near Anchorage, Alaska, when he saw a wave of snow cascading towards him. The terrain was so steep that “it would be like looking up at a 10-storey building and seeing a truck falling off the top towards you”. He knew he would be hit within seconds, but he hoped that, if he could hang on to the mountainside, the wave would pass over him. He dropped down, swung his ice axe into the rock and held on as tightly as he could. The snow piled on top of...