In 1959, nine experienced cross country skiers, led by Igor Dyatlov, were trapped in a snowstorm and decided to set camp and sleep it out.
Later that night, something made all nine people leave their tents in such a hurry that they ripped them open from within. These people stumbled down the slope, in subzero weather, only in their underwear with no socks or shoes. All nine were discovered dead and frozen by a search party about a month later.
Here's where it gets really creepy: three of the nine suffered massive chest and head injuries that could only be caused by forces akin to that of a car crash, but without any external wounds. There were high doses of radioactivity. And if those weren't weird enough: one victim was missing her tongue.
Read more about the Dyatlov Pass Accident, including how mysterious "bright flying spheres" were seen nearby and why the findings of the investigation were classified as secret: Link | Photo Gallery (in Russian) | Wikipedia entry - via SF Gate and Super Punch