The Dyatlov Pass Incident

On February 2nd, 1959, during the cold winter on Kholat Syakhl ("Mountain of the Dead") in Russia, nine intrepid ski hikers decided to do what they do best, which is ski hike, whatever the hell that is. On February 26th, the first of their very dead bodies turned up. Man, who would have thought such a tragedy could strike on "The Mountain of the Dead?"

But it was the discovery of the campgrounds that added the icing to the creepy-as-fuck cake. The ski hikers' tent was shredded. The skiers were scattered around the grounds wearing either very sparse clothing or just their underwear. Three of them were found with crushed ribs and fractured skulls, but no visible defense marks or other signs of a struggle.

Oh yeah, and one of the bodies was missing a tongue.
In case you weren't already on the phone with Mulder and Scully, trace levels of radiation were supposedly found on their bodies. The official statement on what happened was about as vague and ass-covering as possible, saying it was caused by an "unknown compelling force." In laymen's terms this means, "fuck if we know."
The story has become an internet sensation over the years, with many people blaming aliens, and then ghosts, and then the yeti, or possibly all of them working in tandem.