Illustration by Jamie Givens

Avalanches have killed some of climbing’s most luminous stars. In 1979, Willi Unsoeld—who summitted Everest in 1963 as part of the first American expedition—died in an avalanche while leading a winter ascent of Mt. Rainier. In September 1999, a massive avalanche triggered by a serac fall killed Alex Lowe and David Bridges on the flanks of Shishapangma. More recently, an avalanche on Mt. Edgar in China in 2009 killed young alpinists Jonny Copp and Micah Dash along with cameraman Wade Johnson.

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Avalanche danger will always be a hazard for those seeking to climb some of the world’s most sought-after peaks. Here’s a look at some facts about the deadly snow slides.

1. Avalanches have killed 165 climbers in the United States since the winter of 1950- 51. Slides also killed 212 snowmobilers, 291 skiers, and 56 snowboarders during the same decades, according to statistics published by the Colorado Avalanche Information Center. Survival statistics are harder to come by, since avalanches that do not result in death are heavily underreported.

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2. The avalanche responsible for killing the most climbers in the U.S. was triggered by a massive icefall on Mt. Rainier (14,411 feet) on June 21, 1981. Eleven climbers—including one guide—were swept into a crevasse and buried under 70 feet of snow and ice, making it impossible to recover any of the bodies. The remaining 18 members of the party survived.

3. A handful of avalanches outside of the United States have killed more climbers than the one on Mt. Rainer. In April 1972, two avalanches on Manaslu, the eighth-highest peak in the world at 26,758 feet, buried the tents of a sleeping South Korean climbing team at Camp III. In all, 15 people died, including 10 Sherpas and a Japanese cameraman. Three people survived after being carried 2,500 feet in the fi rst avalanche and another 1,000 feet in the second. In 1937, an avalanche wrecked a Nazi-fi nanced German expedition up Nanga Parbat. Seven Germans and nine Sherpas died. In October 2005, a giant avalanche on the Himalayan peak Kang Guru killed 18 members of a French climbing team who were resting inside their tents after afternoon tea. But the deadliest avalanche in mountaineering history was a 1990 slide on Lenin Peak, which sits on the border of Tajikistan and Kyrgyzstan in the Pamir Mountains. An earthquake triggered the avalanche, which buried Camp II, killing 43 people from a team of 140 international climbers.

4. Although climbers and other outdoorsy people often put themselves in harm’s way, the deadliest avalanche in American history involved a train wreck. In the early morning of March 1, 1910, an avalanche roared down Windy Mountain in Wellington, Washington. The wall of snow swept the Spokane Express passenger train—which had been stranded at the Wellington Station for days as a blizzard pounded the region—from the tracks outside the station. The train and the passengers sleeping onboard plunged into a 150-foot-deep ravine, and though some were dug out of the wreckage alive, 96 people died in the accident.

5. Globally, the deadliest avalanche may have happened more than 2,200 years ago, when Hannibal tried to march the Carthaginian army, their horses, and a contingent of three dozen elephants across the Alps from Spain to conquer Rome. After he managed to ferry his troops across the formidable Rhone River (likely relying on pig bladders as flotation devices for his pachyderms), avalanches in the Alps are believed to have killed 20,000 of his men. Hannibal survived the ordeal, but his mission had been thwarted.

6. The most lethal snow slide is known as a slab avalanche, characterized by an entire layer of cohesive snow breaking free and sliding across a weaker layer buried deeper in the snowpack. The avalanche is frequently triggered by added pressure—such as a person or a big dump of fresh snow—causing the weaker snow layer underneath to collapse. Once a slab avalanche has been triggered, the snow can accelerate to between 60 mph and 80 mph in as little as five seconds, according to the Utah Avalanche Center.

7. In 90 percent of avalanche accidents, the victim, or someone in the victim’s party, precipitates the slide. Often, an avalanche is triggered when a person’s body weight provides just enough extra stress to collapse the weaker layer below. These “trigger points” are often at spots where the overlying slab of snow is thinner, such as convex bulges in the slope, near ridgelines, or under rock bands. Contrary to popular belief, noise does not trigger avalanches.

8. The vast majority of avalanches (90 percent) occur on slopes with angles between 30 and 45 degrees. Steeper slopes tend to continually slough snow, keeping a deep snow pack from building up, and the snowpack on flatter slopes requires more force to move. For limbers—whose routes may travel up vertical ice flows or steep couloirs—the greatest avalanche danger often comes during the approach or from above. Climbers should pay special attention to the angle of any snow slopes above them that would funnel avalanche debris down their route if they slid.

9. The primary cause of death among all people completely buried by an avalanche is asphyxiation, though the percentages vary by activity. A study of 204 avalanche fatalities in Canada between 1984 and 2005 found that asphyxia was responsible for 75 percent of the deaths overall, and that trauma was the cause of 24 percent of the deaths. (The last one percent were killed by hypothermia.) But for climbers, trauma was the cause of death in 42 percent of avalanche fatalities.

10. Because asphyxiation is the most common underlying cause of avalanche deaths, the more quickly a victim can be located and dug out, the higher the chance of survival. If a victim can be rescued within 18 minutes, the survival rate is greater than 91 percent, according to a study in the Canadian Medical Association Journal. The survival rate drops to 34 percent in burials between 19 and 35 minutes. Finding a buried avalanche victim quickly typically depends on using avalanche transceivers, which allow the rescuers to home in on the victim by tracking a radio pulse from the victim’s beacon. Climbers, however, are far less likely to use avalanche beacons than skiers. Canadian researchers found that only eight percent of climbers wear beacons in avalanche terrain compared to nearly 100 percent of backcountry skiers.