By the time the rescue team helicoptered to the remote Dyatlov Pass in late February 1959, the nine Russian adventurers—seven men and two women, all highly experienced cross-country skiers—had been dead for nearly a month. Nothing about the scene seemed right. The adventurers’ tent had been sliced open from the inside, and in its husk lay rucksacks, neatly arranged boots, and a plate of sliced pork fat. The rescuers found the victims themselves over half a mile downslope from their camp, some of them barefoot and almost naked. The primary cause of death was hypothermia—temperatures would have been well below zero degrees Fahrenheit the night they fled—but two of the deceased were missing their eyes, and another her tongue. Four had suffered severe trauma to their heads and chests, as if they’d been in a car crash. These were not injuries consistent with a death by avalanche.
Over the decades, what became known as the Dyatlov Pass incident has prompted many a conspiracy theory. It must have been aliens that made the Russians flee to an icy death, as evidenced by the fact that some of the adventurers’ clothes bore traces of radioactivity. Or a Yeti had stumbled upon the camp. Or, more plausibly, the local humans didn’t appreciate the group’s intrusion on their lands. In the end, none of these were particularly convincing to the Russian government, which officially blamed an avalanche as the culprit, all those curious circumstances notwithstanding.
Now, more than 60 years later, scientists say they’ve got new evidence to back up that claim, but with a twist: The killer was probably a peculiar kind of avalanche. Inspired by previous work that modeled realistic snow for the Disney film Frozen, the researchers simulated how a relatively tiny avalanche could have struck the camp, forcing the adventurers to flee, and severely injuring some of them.
Sometime before nightfall on February 1, 1959, the cross-country skiers had made a simple camp, just one tent into which they’d all crammed side by side to sleep. We know this timeline because investigators recovered the group’s cameras and found an image of the pitched tent in daylight. We also know that the group evacuated quickly at night, as evidenced by the cut in the tent—perhaps done frantically, if people trapped inside couldn’t breathe—and the fact that the rescue team also recovered lanterns near the shelter.
As for the campers’ time of death, researchers have a few lines of evidence. Without body heat to keep it warm, in such cold weather—estimated to be -13 degrees F, based on readings from the closest weather stations—a person’s watch stops ticking about an hour after their death. Three of the adventurers’ watches stopped between 8am and 9am, and another’s stopped at 5:31 am. The temperature also determines the maximum survival time for an exposed human being wearing little clothing; in this case, the window should have been two to three hours. Lastly, the victims’ stomach contents showed it’d been six to eight hours since their last meals.
Putting this all together, the skiers pitched camp before nightfall, likely fled between 1:30 am and 5:30 am, and perished between 4:30am and 7:30am.
If you’re envisioning a typical avalanche, this doesn’t make much sense. For one thing, the rescue team didn’t see any sign of a massive movement of snow—they had easily spotted the tent, and it was not deeply buried. Plus, the slope around the tent was 23 degrees on average, well below the 30 degree incline that scientists will tell you it takes to trigger an avalanche. When the adventurers had originally set up camp, they’d cut into the snow to level out a space. Yet investigators determined that the campers hadn’t fled until at least nine hours after making that cut. If the cut had triggered an avalanche, that slide should have happened immediately.
This all does make sense, though, to scientists Johan Gaume and Alexander Puzrin, who laid out their theory for the Dyatlov Pass incident today in the journal Communications Earth & Environment. (It is, incidentally, the first scientific paper I’ve ever read that mentions “attacks by Yetis.”) The conditions, they argue, could well have spawned what’s known as a delayed slab avalanche.
When the year’s first snow falls, it comes in contact with ground that’s still relatively warm. But the air temperature has fallen dramatically, creating a temperature gradient that builds a porous crystalline material, known as a weak layer, that’s 80 percent air. On top of this, more snow falls, forming a denser slab. Think of it like a parking garage, with the weak layer being the parking spaces and the sparse pillars—lots of airy space. The solid ceiling above is the slab. Now if you somehow disturb that weak layer, knocking out those pillars, it’ll collapse, releasing the slab above it as an avalanche.
Or think of what happens when you stack two books on your palm, then tilt them: The top book slides off once it reaches a critical angle. During a slab avalanche, the same thing happens with that top layer of denser snow; it slides right off the lower layer.
Critically for our scenario, a slab avalanche doesn’t require a 30 degree slope to trigger it—it’s more like 20 degrees. The average slope above the adventurers’ camp was 23 degrees, and investigators found that at the time of the Dyatlov Pass incident, the base of the local snowpack was weak.
The cross-country skiers had actually pitched camp on a small step in the hillside, scooping away the snow to level it out. When they cut into the snowpack, they sliced through the weak layer, essentially initiating a countdown. “When you create a cut in the slope to install the tent, it’s like when you remove a retaining wall,” says Gaume, a snow physicist at the Swiss Federal Institute of Technology. The slab of denser snow now hung precariously over the camp. “All the ingredients were there,” Gaume adds. “There was a weak layer, there was a slab, and the slope angle was locally steeper than the critical angle.”
But why did the slab hang there—for between 9.5 and 13.5 hours by the researchers’ calculation, given the campers’ times of death—before crashing through the tent? This is very unlike a typical avalanche, which smashes down immediately after a disturbance, for instance an unfortunate snowboarder. “Very rarely there are cases when you throw explosives into a slope to trigger an avalanche, and then actually the avalanche releases, let’s say half an hour later, or something like that,” says Jürg Schweizer, an avalanche scientist at the Swiss Federal Institute for Forest, Snow and Landscape Research, who wasn’t involved in this new work. “And then those avalanches are typically explained by saying, ‘Well, maybe you caused a subcritical failure and that then developed or grew within some time to critical size for crack propagation.’”
The delayed slab avalanche in question is a different beast. The weak layer around the camp was apparently still strong enough to initially support the snow for several hours, but there was another factor at play. Gaume and Puzrin knew from investigations of the incident that at the time weather stations had detected strong “katabatic” winds, a phrase taken from the Greek, meaning “to go down.” These winds do exactly that: Pulled by gravity, they descend rapidly from the tops of hills and mountains, scouring snow from higher elevations and depositing it below.
The researchers modeled how such winds could have built up snow above the tent, and how long it would have taken to reach a critical load that would cause the top slab to slip off the weaker layer below, now that its structural integrity was compromised by the cut. “This was how the loading was increasing,” says Gaume. “Because there was no other way—there was no snowfall on that night.” Sometime after midnight, enough weight had built above the weak layer that it suddenly collapsed, sending the slab into the tent. It would have been a relatively small avalanche—maybe 16 feet by 16 feet—which the researchers simulated with inspiration from the Disney snow model. It would have been enough to fill the hole the campers had dug into the snow, but not enough that the rescue team would be able to find clear signs of an avalanche 26 days later.
An avalanche doesn’t need to be large, though, to cause grave damage to the human body. Typically, hikers who get caught up in one are likely to just suffocate. But in this case, none of the nine victims died of suffocation, and some had severe chest and head trauma.
This, too, can be explained by the dynamics of the slab avalanche and the downward winds. While it wasn’t snowing at the time of the incident, the katabatic winds would have produced a much more dangerous kind of deposit above the tent. “The wind was eroding and transporting the snow, which was made of very small crystals,” says Gaume. “And then when it deposits, [the crystals] are highly compacted.” This could have created a dense slab of snow that weighed perhaps 25 pounds per cubic foot. And even more unfortunate for our adventurers, they’d laid their skis out as a floor for their tent, creating a hard substrate for the snow to crush them against.
Gaume and Puzrin went even further by modeling what this trauma could have looked like. To calibrate their simulation, they used data from old automotive industry crash tests done using human cadavers, rather than dummies. (To be fair, it was the 1970s, which was a … different time.) They then modeled the release of simulated snow blocks of different sizes onto a digital model of a human body, and compared that to the crash test results. “What we saw is that it would not be fatal, but it would create moderate-to-serious injuries,” says Gaume. (Below, you can see the damage a chunk of snow a meter across could do.)
From this, they concluded that the mountaineers survived the initial crush of snow, cutting their way out of the tent, although some of them were seriously injured. But if they’d escaped a relatively small avalanche, why would they flee over half a mile away, instead of sticking around to dig out their supplies, especially their boots? Investigators found the group had actually stashed another set of supplies in the forest, so perhaps they’d set out for them in a panic. “You start to cut the tent from the inside to get out,” says Gaume. “You see there was an avalanche, and then you might be afraid of a second avalanche. And so they may have decided that the best option would probably be to go to the forest, make a fire, and try to find the supply.”
But, clad in little clothing, they didn’t survive more than a few hours in the biting cold. They had no way of knowing the intricate dynamics of a slab avalanche, and that, shocked as they were, it may have been safe to dig out their supplies and move along.
“It really does look to me that an avalanche is the most plausible explanation of the situation,” says Jordy Hendrikx, a snow avalanche scientist at Montana State University, who wasn’t involved in this new work. “I think it’s exciting that someone like Johan [Gaume], with the skill set that he has and the way that he’s developed his models, can give us some more insights on that.”
Back in 1959, investigators had a more limited understanding of how avalanches work. They didn’t realize, for instance, that under the right conditions, it’s perfectly reasonable for an avalanche to slide on a slope of less than 30 degrees. So for a while, authorities suspected local peoples of murdering the adventurers. And if an avalanche didn’t seem particularly plausible, why not consider the supernatural? “I think aliens and Yeti are fun ideas,” says Hendrikx. “And especially given the time, the Soviet Union and, even more recently, Russia, aren’t known for transparency in their information. So I can understand why these wild theories have come together.”
While this new slab avalanche theory actually has scientific rigor to it, it remains exactly that: a theory. After all, no eyewitnesses survived. “I want to be clear that we do not claim that we solve the mystery,” says Gaume. “I mean, no one wants this mystery to be solved in Russia. This is part of the Russian lore.”
This story originally appeared on wired.com.