One human footstep can start it. Millions of tons of snow and ice, crushing everything it its path. Avalances are unpredictable, unstoppable killers. ANd they're not made of just snow. Volcanoes like Japan's Mt. Fugen produce dealy pyroclastic floows of ash, rock and poison gas. But snow avalanches - a million each year - pose the greatest danger. Watch a powder avalanche hurtle down a mountain at 200 mph, smashing trees like matchsticks. Hang on as skiers race for their lives fore rivers of snow. Meet avalanche controllers in Canada's Bear Pass, who use a 105 mm canon to safely trigger slides. And climb aboard a helicopter to bomb unstable snowpacks with high explosives. You'll never look at a snow-covered peak the same way again. -- On back of video jacket.