Avalanches occur naturally in the mountains, when the snowpack becomes too unstable, but can also be triggered by human activity – on purpose or by accident. Once the snow starts sliding, it becomes an extremely powerful, destructive, and deadly mass of energy, swallowing everything in its path. On December 28, 2008, this is just what happened in Sparwood, a mining town in British Columbia. Friends out for a fun day of snowmobiling are in the crosshairs of not one, but three successive avalanches.