Photo courtesy Jim Krueger

Hiking or skiing in avalanche country is like walking in a valley inhabited by grizzly bears: Your senses become more alert. You become aware of tiny sounds—every creak of a tree limb, every snap of a twig. When each footstep on a steep slope is potentially your last, you tend to pay attention to where you put your feet. Time slows down. Your actions matter.

Once a slab of dense snow becomes sufficiently unstable, it can begin to slide downhill with almost unimaginable force. Small avalanches can carry impact pressures of up to 1,000 pounds per square foot, enough to completely demolish a wood-frame house; larger slides, with 20,000 pounds per square foot, can crush a concrete building. Researchers have estimated that an average powder-cloud avalanche of 160,000 tons can generate 20 million horsepower, about 2,857 times that of an Amtrak locomotive. Getting caught in an avalanche is like trying to stand on a breaking wave. It both violently tumbles you forward and sucks you under.

In January 1951, after a two-day blizzard dropped 38 inches of snow on Alta, Utah, avalanche expert Monty Atwater was checking the slopes before turning skiers loose. At a steep, concave chute his ski patrol partner triggered an avalanche, and Atwater fell through the cascading snow until his skis hit the hard base of snow underneath. "I was knee deep in boiling snow, then waist deep, then neck deep," he wrote in his 1968 memoir, The Avalanche Hunters. "Very fast and very suddenly I made two forward somersaults, like a pair of pants in a dryer. At the end of each revolution the avalanche smashed me hard against the base. It was like a man swinging a sack full of ice against a rock to break it into smaller pieces.

"My principal sensation was one of wild excitement," he wrote. "Under the snow there was utter darkness instead of that radiance of sun and snow which is never so bright as directly after a storm. It was a churning, twisting darkness in which I was wrestled about as if by a million hands. I began to black out, a darkness that comes from within. Suddenly I was on the surface again, in sunlight. I spat a wad of snow out of my mouth and took a deep breath.... The next time I surfaced I got two breaths. It happened several times: on top, take a breath, swim for the shore; underneath, cover up, curl into a ball. This seemed to go on for a long time, and I was beginning to black out again. Then I felt the snow cataract begin to slow down and squeeze.... I gave a tremendous heave, and the avalanche spat me onto the surface like a seed out of a grapefruit."

There are, in fact, several ways to die in an avalanche. A third of the fatalities are caused by trauma to the head and neck sustained during the fall, from smashing bones on rocks buried in the snow or from the contortions inflicted on a body by cascading snow. The rest are due to suffocation. Even with a small air pocket, the warmth of a victim's breath can seal the snow around his mouth much as perspiration seals the inside of an igloo or a snow cave. Within minutes, a virtual mask of ice forms around the face, cutting off any flow of air.

Few experiences are more terrifying. "Try as I did, it was absolutely impossible to expel the snow from my mouth," a survivor named Bill Flanagan told ava-lanche researchers Betsy Armstrong and Knox Williams. "The ball of snow simply packed harder each time I tried to gulp air around it." When he finally came to a rest, the ice ball in his mouth "was so big and hard that I was unable to get it out from behind my teeth. I was able to crush it bit by bit with my front teeth and finally reduced it to a size I could at last spit out."

With so little oxygen available under snow, avalanche survival depends critically on the efforts of survivors to dig out their compatriots. There just isn't time to wait for rescuers. And when all the members of an expedition get buried, the chances for survival are very slim.