In 1942, Indian forest ranger H.K. Madhwal came across something rather disturbing: a lake filled with bones. Hundreds of people died in the lake, all at once. But what killed them?

Roopkund Lake is located in the state of Uttarakhand, India. The lake sits at over 16,000 feet in elevation, and it freezes over—so the skeletons are only visible during a brief thaw. Madhwal found it at just the right time to see the bones. But how old were they? And what were they doing hanging around in a lake?

Let Dylan Thuras of Atlas Obscura tell you all about the mystery...and its surprising solution.