On the surface, what’s become known as the Dyatlov Pass incident seems fairly explicable: Of a party of ten skiiers, nine perished in the middle of a high-difficulty trek in conditions that reached -30 degrees Celsius. But the details, which are mostly based on diaries of those involved as well as records from Soviet investigators, are chilling: On the night of February 2, 1959, members of the party apparently ripped their tent open from the inside, and wandered into the tundra wearing nothing but what they wore to bed.

Three weeks later, five bodies were found, some hundreds of meters down a slope from the original camp. It took two more months for investigators to find the other four bodies, which, curiously, were partially clothed in articles belonging to the earlier-discovered dead. Tests of those clothes found high levels of radiation. Despite that, and heavy internal trauma, including fractured skulls and broken ribs, suffered by some members of the party, Russian investigators reported they could not find evidence of foul play, and quickly shut the case.

The group was made up of students and graduates of the Ural State Technical University, all of whom were experienced in backcountry expeditions. The trip, organized by 23 year old Igor Dyatlov, was meant to explore the slopes Otorten mountain in the nothern part of the Ural range, and started on January 28, 1959. Yury Yudin, the only member of the expedition to survive, got sick before the crew made it fully into the backcountry, and stayed behind at a village. The other nine trekked on, and according to photographs developed from rolls recovered by investigators, Dyatlov’s crew set up camp in the early evening of February 2 on the slopes of a mountain next to Ortoten.

That mountain is known to the local, indigenous Mansi tribe as Kholat Syakhl, which supposedly translates to “mountain of the dead,” although with a tale like this, I’d take something so perfectly creepy with a grain of salt. Still, the decision to camp on the mountain’s slope makes little sense. The group was reportedly only about a mile from the treeline, where they could have found at least a bit more shelter in the subzero conditions. They didn’t appear to be strapped for time, and setting up camp on the face of a mountain rather than within a nearby forest is questionable, although not indefensible.

That camp would be the group’s last. Dyatlov had previously said that the team expected to be back in contact on February 12 of that year, but also said that the group might take longer than expected. It wasn’t until around the 20th that the alarm was raised, and by the 26th the camp had been found by volunteer search and rescue teams.

When official investigators arrived, they noted that the tents appeared cut apart from within, and found footprints from eight or nine people leaving the tents and heading off downslope in the direction of the treeline. According to investigators, the group’s shoes and gear were left behind, and the footprints suggested some people were barefoot or wearing nothing but socks. In other words, they all shredded their way out of their tent and ran off through waist-deep snow in a huge hurry, despite there being no evidence of other people or foul play within the group.

The first two bodies were found at the treeline, under a giant pine tree. Remember that the treeline was about a mile away; investigators wrote that footprints disappeared about a third of a way there, although that could have been due to weather in the three weeks it took for investigators to arrive. The two bodies found were both wearing only their underwear, and both were barefoot. According to reports, branches were broken high up the tree in question, which suggested someone had tried to climb it. The remains of a fire lay nearby.

Three more bodies, including Dyatlov’s, were found at points in between the camp and the big tree, and apparently lay as if they were headed back to the camp. One of them, Rustem Slobodin, had a fractured skull, although doctors declared it non-fatal, and the criminal investigation was closed after doctors ruled the five had died of hypothermia.

Two months passed until the remaining four bodies were found buried under a dozen feet of snow in a gully a few hundred feet downslope from the big tree. The inexplicable behavior of the prior five members of the party aside, it was the discovery of this quartet that was most horrific. All four suffered traumatic deaths, despite there being no outward appearance of trauma. One, Nicolas Thibeaux-Brignollel, also had a fractured skull. Alexander Zolotariov was found with crushed ribs. Ludmila Dubinina also had broken ribs, and was also missing her tongue.

It is possible that the group was searching for help–despite being in, essentially, the middle of nowhere, while missing gear in sub-zero temperatures–before they fell into a ravine. But that doesn’t explain Dubinina’s missing tongue. And while some at the time posited that the group had been attacked by Mansi tribesmen, coroners at the time stated that the trauma found required more force than humans could inflict, especially considering there wasn’t accompanying outward trauma.

It gets weirder. The final four were better outfitted than the other five, and apparently had taken clothes off the dead as they continued their aimless trek. Zolotariov, for example, was found wearing Dubinina’s coat and hat, while she in turn had wrapped around her foot a piece of the wool pants that one of the two found at the pine tree had been wearing. To add to the mystery, the clothes found on the final group were tested and found to be radioactive.

After the group’s bodies were discovered, an investigation by Soviet Union authorities determined that six had died from hypothermia while the other three showed signs of physical trauma. One victim had a fractured skull; two others had major chest fractures. Additionally, another team member was missing her tongue and eyes. The investigation concluded that an “unknown compelling force” had caused the deaths. Numerous theories have been put forward to account for the unexplained deaths, including animal attacks, hypothermia, avalanche, infrasound-induced panic, military involvement, or some combination of these. The theories go on and on but truth be known, we really don’t know what happened to these people.