The internet has no shortage of mysteries for those inclined to chase them. Many fit neatly within genres with dedicated fan bases: true crime, Bigfoot, unexplained lights in the sky. But some are so inexplicable, so disturbing, that they worm their way into nearly every corner of the internet’s ever-rolling theory-machine. These mysteries seem to not only defy explanation, but they even befuddle many of those who are quick to attribute such events to the supernatural.

One such incident took place in the unforgiving wilderness of Russia’s Ural Mountains in January of 1959. Ten experienced hikers—nine students from the Ural Polytechnical Institute along with one older ski instructor—set out to conquer Otorten Mountain, a peak in the northern portion of the mountain range. They began their journey by train, traveling to the Soviet city of Ivdel. There they boarded a bus that took them to Vizhay. From there, they took a truck even farther into the frigid forest, arriving in an area known only to local loggers as “Sector 41.”

From there, a local logger led the intrepid group to an abandoned geological site known as “Nord 2,” where they made camp for a day. There, in the last bit of truly “settled” territory they would come across for days, one of the hikers, Yuri Yudin, fell ill. Over the course of the following day, Yudin took it easy, collecting rock samples for the Institute of Mining back at Sverdlovsk and hoping his illness was nothing but a temporary setback. The following day, however, Yudin acknowledged that his health wouldn’t allow him to continue the trip, so he parted ways with the nine remaining hikers as they set off on their way. That was the last time Yudin would see any of his friends alive.

The remaining nine hikers, led by experienced mountaineer Igor Dyatlov, set out on their trip and, according to photographs and journal entries discovered later, were in high spirits despite some nasty weather and the difficulty of covering miles through deep snow. They snapped photographs along the way, most of which look like they could have been taken by any group of college kids on such a wilderness adventure.

Then, on Feb. 1, the tenth day of the hike, Dyatlov’s party stopped and made camp on the eastern ridge of a mountain known only as “Height 1079.” According to some reports, they found their way there as a result of getting lost due to low visibility caused by the winter weather, and for some reason, chose to establish their camp on the slope of the mountain.

The area’s indigenous population, a tribe that call themselves the Mansi, have a different name for that mountain: “Holatchahl.” Mountain of the Dead.

Had they walked only about a mile farther, they would have found flatter ground and more natural protection from the elements. Why Dyatlov chose that area remains a mystery. Some contend that he didn’t want to sacrifice the altitude so they could begin the next morning with a downhill hike. Others say he may have wanted to practice mountain slope camping. In any event that decision may have cost the entire party their lives.

Before the sun rose again over the Ural Mountains, all nine members of Dyatlov’s party were dead. Objectively, the deaths of nine adventurers on a frigid, snow-covered mountainside in Russia may sound tragic, but is not unusual. These types of journeys are inherently dangerous–which, one could contend, is why they offer such allure. But it wasn’t that the hikers all died that earned the Dyatlov Incident its place among the greatest mysteries of the Soviet era–it’s how they died that continues to prompt new theories to this day.

The bodies of Dyatlov’s party weren’t discovered until weeks later. Once relatives began to worry, they notified Soviet authorities, who mounted a search and rescue operation on Feb. 26. They soon found the climbers’ abandoned tent still secured to the side of the mountain slope.

Investigators immediately noted that the tent had been cut open from the inside, with nine distinct sets of tracks leading out of the tent from the open hole. Some of the tracks were even barefoot, indicating that Dyatlov’s party fled the tent in a panic, with many leaving their jackets, gloves, and even boots behind. Some suggested that the tent may have caught on fire, prompting the rapid escape–but no signs of a fire were found.

The investigators then followed the tracks leaving the tent down the mountain slope and into the nearby forest. About a third of the way down the slope, investigators noted, the footprints disappeared, which they attributed to snowfall following the incident. About a mile from the tent, they found the remains of an improvised campfire and the first two bodies. They were wearing only their underwear, with nothing on their feet. A nearby tree had a number of branches broken, some fairly high up the trunk, leading some to speculate that one of the two may have attempted to climb the tree to escape something below or to gain a higher vantage point.

Soon, three more bodies were found at different locations between that impromptu camp and their tent. These bodies, which included Dyatlov, appeared to be headed back toward the tent they had fled. Unlike the first two, who appeared to have simply died of hypothermia due to exposure to the elements, these bodies were in far worse condition.

Rusem Slobodin, for instance, was found with a fractured skull, though the coroner that examined him contended that the injury would not have been fatal. Instead, it seemed likely that the injury had occurred prior to him succumbing to hypothermia.

The four other bodies were not discovered until nearly two months later. Well into the spring, after nearly a dozen feet of snow had melted, these bodies were discovered in a gully about a hundred feet from the impromptu camp. Initially, they didn’t appear to have suffered much in the way of trauma, and speculation was that they too had simply died from the cold. That is, until autopsies were conducted.

All four of the bodies showed signs of an extremely traumatic death. Nicolas Thibeaux-Brignollel, like Rusem Slobodin, had a fractured skull. Alexander Zolotariov and Ludmila Dubinina had their ribs either broken or crushed inward. Dubinina’s tongue had been removed from her mouth, while the other two were missing their eyes. According to one of the doctors examining the bodies, the force exerted on Zolotariov’s rib cage would have been equal to that of a car crash. This might suggest that an avalanche was the culprit, but there were no signs of avalanche elsewhere on the slope: The tent remained in place and the snowfall appeared natural. Even stranger, these four dead hikers were found far better equipped than their peers. Some of them were found wearing the clothes of other hikers, with one using spare clothing to wrap her feet and create makeshift shoes.

If that doesn’t seem quite strange enough, testing conducted at the site and again during the autopsy revealed that the clothing these four were wearing was radioactive. Soon, local reports of lights in the sky around the area of the Dyatlov party’s demise were finding their way to investigators, who, under orders from the Kremlin, promptly sealed the case files and declared the investigation closed. Officially they only acknowledged that the party had been killed by ” an unknown compelling force which the hikers were unable to overcome.”

According to statements made by investigators at the scene, once they were prompted to check, their Geiger counters registered extremely high levels of radiation in the area. Also, one of the camper’s friends reportedly claimed that their bodies were “deeply tanned” at their funerals.

Another group of campers located some 30 miles from Dyatlov’s party on that same night reported “a shining circular body flying over the village from the southwest to the northeast. The shining disc was practically the size of a full moon: a blue-white light surrounded by a blue halo. The halo flashed brightly like the flashes of distant lightning. When the body disappeared behind the horizon, the sky lit up in that place for a few more minutes.”

The lead investigator on the case, Lev Ivanov, has even gone on record to say that he does not agree with the formal conclusion of the Soviet Union that the incident was simply an avalanche.

“I suspected at the time and am almost sure now that these bright flying spheres had a direct connection to the group’s death,” he told the newspaper Leninsky Put years later.

Since then, everything from mountain curses to UFOs have been floated as potential explanations. Some believe the Dyatlov party encountered a UFO. Others contend that they were attacked by the Abominable Snowman. Many others contend that the Soviet government’s behavior following the incident, along with reports of lights in the sky, suggest some sort of nuclear weapons testing gone terrible awry. A nuclear-powered cruise missile, not unlike those being tested by the Russian government today, could potentially be responsible for irradiating a swath of forest and sending a group of hikers into a frenzied panic.

The Russian government has recently reopened its investigation into the fate of the Dyatlov party. Though its statements have made it clear that the intent is to prove that the deaths were caused by an avalanche, rather than to unearth any new evidence. This has prompted some to suggest that the cover-up—for whatever reason—remains ongoing.