Avalanche, just the word makes your spine cold. In a Wikipedia entry:
… Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces on the snow exceed its strength but sometimes only with gradually widening (loose snow avalanche). After initiation, avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow. If the avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current. The load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches that occur in this way are known as spontaneous avalanches. Avalanches can also be triggered by other loads such as skiers, snowmobilers, animals or explosives. Seismic activity may also trigger the failure in the snowpack and avalanches.
In Afton, Wyoming, a snowmobiler found out how quickly an avalanche can occur and why those using snowmobiles should exercise caution when traversing snow-covered mountainous regions. The loud noise alone could cause an avalanche.A video shot from a helmet camera was posted at YouTube and fortunately other snowmobilers were present to witness the avalanche and rescue the victim on the slopes. The force of the avalanche knocked him off his snowmobile and his helmet. Luckily, a ski from the vehicle protruded through the snow marking where he was buried. Avalanche victims usually die from suffocation.