In February 1959, a search was organized when nine Russian ski hikers failed to return from a trek in the northern Ural Mountains. After six days, their abandoned camp was found in a mountain pass.

All the hikers were dead. Two were found on the opposite side of the pass, near the remains of a fire; three others had died closer to camp, apparently trying to return; and the remaining four were found only three months later, under 4 meters of snow in a nearby stream valley.

Apparently the victims had fled the tent suddenly on the night of Feb. 2, tearing their way out from the inside and running down the mountain. Though the temperature had been around -25° C, all were inadequately dressed, some wearing only underwear. Though the bodies had no external wounds, one showed severe skull damage and two had major chest fractures. One woman’s tongue was missing.

In the end, Soviet investigators could conclude only that a “compelling unknown force” had caused the hikers’ deaths. That’s all that’s known.