Excerpted from SFGate.com

In 1959, nine experienced Russian cross-country skiers — seven men and two women, led by a man named Igor Dyatlov — headed to the Ural Mountains, to a slope called Kholat Syakhl (Mansi language for "Mountain of the Dead,") for a rugged, wintry trek. On their way up, they are apparently hit by inclement weather and veer off course and decide to set up camp and wait it out. All is calm. All is fine and good. They even take pictures of camp, the scenery, each other. The weather is not so bad. They go to sleep.

Then, something happens. In the middle of the night all nine suddenly leap out of their tents as fast as possible, ripping them open from the inside (not even enough time to untie the doors) and race out into the sub-zero temps, without coats or boots or skis, most in their underwear, some even barefoot or with a single sock or boot. It is 30 degrees below zero, Celsius. A few make it as far as a kilometer and a half down the slope. All nine, as you might expect, quickly die.

And so it begins.

Why did they rush out, unable to even grab a coat or blanket? What came at them? The three-month investigation revealed that five of the trekkers died from simple hypothermia, with no apparent trauma at all, no signs of attack, struggle, no outward injuries of any kind. However, two of the other four apparently suffered massive internal traumas to the chest, the kind you sustain in a car accident. One's skull was crushed. All four of these were found far from the other five. But still, no signs of external injuries - except that one of the women was missing her tongue.

It gets better. And weirder.

Tests of the few scraps of clothing revealed very high levels of radiation. Evidence found at the campsite indicates the trekkers might've been blinded. Eyewitnesses around the area report seeing "bright flying spheres" in the sky during the same months. And oh yes, relatives at the funeral swear the skin of their dead loved ones was tanned, tinted dark orange or brown. And their hair had all turned completely gray.

The final, official explanation as to what caused such bizarre behavior from otherwise well-trained, experienced mountaineers? An "unknown compelling force." Indeed.

Here's the problem: All the convenient, logical explanations — avalanche, animal attack, secret military nuke test — fail. Russian authorities held a three-month investigation. Rescuers, experts picked through every piece of evidence. There were no signs of natural disaster. And if it was just an avalanche, why was the area closed off for three years following the event, and all related documents put in a secret Russian archive until 1990? If it was some sort of weird nuclear megablast (which may tint you orange, but won't turn your hair gray), what the hell happened to her tongue?

These questions are still unanswered today. If you would like to dive deeper into the facts and mysteries about this story, I recommend you read the St. Pete Times article that covers possible explanations ranging from murder by the indigenous Mansi people to UFO/alien involvement....which are all unsubstantiated, of course...