On January 23, 1959, 10 hikers set out for a winter trek through Russia’s Ural Mountains. One turned back after several days for medical reasons, but the other nine continued along their route. They had planned to send a message back to their sports club about three weeks after taking off; so when the club didn’t hear from them by February 20, a search party set out to find the hikers.

Over the next couple of weeks, the search party found the first five of the hiker’s bodies spread out over the snow. They were in various states of dress and had bizarre injuries, with one appearing to have bitten off part of his own knuckle.

Months later, after some snow melt, investigators discovered the bodies of the remaining four hikers. They had even more inexplicable injuries. One had a fractured skull, another had a twisted neck, two were missing their eyes and one of the bodies with no eyes was also missing her tongue.

The gruesome fate of the nine hikers has generated theories ranging from natural disasters to secret weapons testing to an attack by yetis. Although some theories are more plausible than others, the “Dyatlov Pass incident,” as it is known, remains a contentious and unsolved mystery.

The Dyatlov Pass—the section of the Ural Mountains that the hikers were trekking to—is named after the leader of the fateful expedition: Igor Dyatlov. Dyatlov was a 23-year-old student studying engineering at the Ural Polytechnic Institute in Soviet Russia. He planned the trip with eight other students at the institute who were in their early 20s. All of the students were experienced hikers, as was the sports instructor in his late 30s who joined the expedition.

About five days into the journey, student Yuri Yudin decided to leave the hiking team and head back due to a sciatica flare-up. If he had stayed with them just a few days longer, he might have died in whatever accident killed them. That’s because around February 1, the nine hikers pitched their tent for the last time.

When the search party found the tent, it was collapsed and covered in snow. Inside, the hikers’ belongings were relatively undisturbed. The team found the hikers’ boots, clothes and equipment neatly arranged in the tent, and food was sliced up on a plate as if the hikers were preparing to eat it. The tent was slashed open and, as a seamstress later observed, someone had made the cut from the inside.

The first two bodies the search party found were those of students Yuri Doroshenko and Yuri Krivonishchenko several hundred yards away from the tent. They were lying in their underwear next to the remains of a fire. A medical examiner later noted that Krivonishchenko had burns on his body and a piece of flesh in his mouth that he had bitten off of his own hand.

The searchers found the bodies of hiking leader Dyatlov and Zinaida Kolmogorova, one of the two female students on the hike, in a different location that suggested they were trying to head back to the tent. Later, searchers found the body of Rustem Slobodin, who also appeared to be trying to make his way back to the tent when he died.

All five students appeared to have died from hypothermia, which can cause erratic behavior and “paradoxical undressing,” a phenomenon in which people start to undress because they feel hot even though they are freezing to death. But there were still four bodies missing.

That May, when more of the snow in the area had melted, an Indigenous Mansi hunter discovered the remains of what appeared to be a snow den. Inside, rescue workers found the bodies of Aleksander Kolevatov, Nikolay Thibeaux-Brignolle, sports instructor Semyon Zolotaryov and Lyudmila Dubinina, the second female student on the trip.

For three of these bodies, the cause of death didn’t seem to be hypothermia. Thibeaux-Brignolle had a skull fracture so severe there were pieces of bone in his brain, while Zolotaryov and Dubinina had crushed chests. Both Zolotaryov and Dubinina’s eye sockets were empty, and Dubinina was missing her tongue.

The initial investigation into the deaths couldn’t determine that a crime had taken place, and ended up concluding that the hikers had died from a “compelling natural force.”

But some Russian citizens wondered if the Soviet government was covering something up. The discovery that some of the hikers’ clothing contained traces of radiation fueled speculation that the hikers had died in some kind of weapons testing disaster.

This radioactivity on the clothes could be explained by the fact that two years before, there had been a nuclear incident known as the Kyshtym disaster. One of the hikers on the trip had lived in the contaminated zone, and another had helped with the clean-up. Despite this, many Russians continued to suspect that the government was hiding something. Theories about what happened have included yetis, the KGB and the CIA.

In 2019, the Russian government reopened the Dyatlov Pass incident. Andrei Kuryakov, the prosecutor who led the new investigation, proposed a slab avalanche theory. The slope on which the hikers planted their tent wasn’t steep enough to suggest there had been a traditional avalanche, but Kuryakov theorized that it was still possible for a slab of snow to slide out over the tent.

This might have caused the hikers to fear that a larger avalanche was imminent, and it could explain why they would cut their way out of the tent and run away without grabbing their boots. Kuryakov speculated that the hikers ran to what they thought was a safe distance from the tent, then started a fire and dug a snow den. The snow den could have collapsed on the hikers who were inside, causing the severe injuries found on only the bodies in that area. Scavenging animals could have eaten the missing eyes and tongue. Those outside the tent would’ve died from the cold.

After Kuryakov released his theory, two researchers in Switzerland decided to test it. In 2021, Johan Gaume and Alexander M. Puzrin published a paper in Communications Earth & Environment that used a mathematical model to determine if a slab avalanche was possible given the location of the tent and the weather conditions at the time. They concluded that it was indeed possible. Whether it really happened that way is another matter.

“We do not claim that now we have a final explanation of what happened,” says Puzrin, who is a professor of geomechanics and geosystems engineering at ETH Zürich. “But we added plausibility to the avalanche theory.”

“What they’re describing is possible,” says Ethan Greene, director of the Colorado Avalanche Information Center. “Is it likely? That’s pretty hard to tell.”

Jim Mcelwaine, a professor of geohazards at Durham University, points out that slab avalanches usually occur at steeper inclines than the one in the Dyatlov Pass incident.

“It’s unlikely, but that doesn’t stop it being the best explanation,” he says. “I don’t know what else it could be.”