Contrary to legend, you can’t start an avalanche by yelling or yodeling, but almost anything that deforms the snow can set one off: falling rocks, melting ice, rain, hail, an earthquake, a footstep. In nine out of ten cases, victims trigger the avalanche that kills them.

Avalanches can take various forms—ice, loose powder, heavy wet snow, rock and glacial flows—but the terrain the K2 climbers traversed on the morning of August 2, 2008 was ripe for a dry-slab avalanche. Climbers on the mountain reported that each time they stepped on the snow, they heard a distinctive creak, and cracks shot across the snow’s surface. They were climbing down a slope of about 40 degrees, an angle well within the 25- to 45-degree range that’s common with dry slabs. Snow had been piling up for weeks, and the temperature had spiked over the previous few days, helping loosen layers of snow.

Skilled climbers will notice these danger signs, but predicting avalanches is imprecise even with the most sophisticated equipment. The chances of a flow depend on the snow’s stickiness, the size and density of the ice crystals, how well those crystals are bonded, the steepness of the slope, the shape of the terrain, the temperature, the humidity, the location and force of the trigger. In broad terms, a slab avalanche occurs when a top layer of snow slips over a lower layer. Anything that makes the space between the two layers more slick (such as watery or ball-bearing ice crystals), or anything that adds pressure on the top layer (such as more snow) increases the likelihood of an avalanche.

Dry snow avalanche with a powder cloud

Many people imagine an avalanche as being a lot of loose snow and ice tumbling down the mountain like a bunch of BBs rolling down a slide. When an avalanche starts, it’s more like a plate sliding off a table. At first, a slab of snow breaks free from the mountain and moves down the slope. As it picks up speed, the slab shatters, breaking into increasingly smaller pieces that eventually become so tiny they flow like water. The material at the bottom of an avalanche is as fine as powdered sugar. Most avalanches flow at around 70 miles per hour; the big ones can reach 200 miles per hour and flow on for miles, washing up and down hills and valleys and striking with enough power to take out trees, houses, and entire towns.

The last, deadly avalanche of the day began all at once, with an enormous, thundering crack. Karim Meherban (a Pakistani high-altitude porter) would have known what the sound meant. The men had about a second and a half to get off the snow slab. That wasn’t enough time.

A moment after the crack, the slab slid out from under them. Within a second, the snow would have been moving at about 10 miles per hour, breaking into giant pieces. Two seconds later, the avalanche would have been sliding between 10 and 30 miles per hour, with chunks further fragmenting. Faster-moving snow at the surface of an avalanche carries more force than the slower-moving snow below it, causing a tumbling motion. For the next five seconds, the slide accelerated, the snow churning like the surf after a wave breaks. At this point, the men would no longer have known which way was up. When this happens in water, surfers sometimes call the condition “being washing-machined.”

The snow, mixing with air, packed into the climbers’ lungs and plugged their mouths, ears, and noses. Their goggles, hats, and mittens were ripped off. “Big” Pasang Bhote and Jumik Bhote were short-roped together (Bhotes are a Tibetan ethnicity culturally distinct from Sherpas). They spooled and threaded around each other, becoming tangled ever more tightly. This apparently broke their necks. Around 3 p.m., Pemba Gyalje Sherpa saw Big Pasang and Jumik tumble past him. They were dead at 3:10 p.m. when he photographed their bodies, tightly wound together in ropes. The snow around them was streaked with blood and tissue.

The flow would have reached its maximum speed, somewhere between 40 and 80 miles per hour, after roughly eight seconds. And then it would have begun to slow. Once it did, it probably took less than a few seconds to stop. The other two climbers were never found, suggesting that the avalanche sucked them down and buried them. The snow probably would have cushioned them so that they remained conscious.

A trained climber would have tried to clear a space around his face, creating an air pocket before the slide halted completely. Then he’d have flailed out his arms and legs so his body would be easier to find.

Once the flow stopped, the snow would have compacted so tightly around him that he couldn’t move even his fingers. Spitting to see which direction was up wouldn’t have helped. The snow feels like concrete, too hard to dig without a shovel. In that situation, all a climber can do is wait, hope, and cough out the snow in the lungs, trying to relax and consume less oxygen.

More than enough air can diffuse through densely packed snow to keep a human alive, but warm breath causes the snow around the face to melt. Inevitably, that melting snow refreezes. This forms a capsule of ice around the climber’s head, preventing fresh air from cycling through. As a result, he is forced to inhale and exhale the same air, with increasingly lower concentrations of oxygen. The climber, buried alive, slowly asphyxiates.

During asphyxiation, the heart initially beats faster. Breathing speeds up. People revived from this state commonly recall seeing a ray or tunnel of light. Many consider it a religious experience. Scientists have an explanation as well, but it hasn’t been tested in a laboratory: Oxygen deprivation causes peripheral vision to decline, narrowing the field of view and giving the illusion of an ever-contracting tunnel of light. Survivors have described it as heavenly.

After about four minutes of asphyxiation, the brain goes into a manic version of REM sleep. Some researchers believe that this brain-wave pattern delays damage to neurons. Victims revived from these moments often remember seeing their entire lives flash before their eyes. They report feeling relaxed, falling into a Zen-like trance that has been known to turn atheists into believers.

After that, the heart, starved of oxygen, slows; the pulse drops to roughly thirty beats a minute. Then the heart beats erratically and soon stops completely, quivering in place, jellylike. Breathing slows, then ceases. The body cools. Electrical activity in the brain diminishes and the central nervous system gradually shuts down.

If the climbers buried by the avalanche didn’t form an air pocket in front of their mouths, they died within thirty-five minutes. With an air pocket, death could have taken about ninety-five minutes. If their bodies cooled quickly, they might have survived for hours in a state of suspension between life and death; hearts stopped, brains partially on, they could be summoned back. Doctors at a state-of-the-art hospital might have been able to revive them.

But the men buried by the K2 avalanche on August 2, 2008 were never found. Their bodies stayed interred, cooling beneath the snow.

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