World War II was one of the first truly global wars, and soldiers in every country were told to be on the look-out for evidence of strange goings on. Even forest guards in Roopkund, India in 1942 were making long treks around the Himalayas and keeping their eyes peeled for anything sinister. One man, walking along a route, came upon a smallish lake. It was set into a round ravine in the mountains, with steep slopes heading down into it on all sides. It looked like water had collected in the bottom of a huge bowl. The slopes were navigable, and so the man made his way down to the lake - and discovered it was full of skeletons. Hundreds of bodies lay below the surface.
The man reported on what he saw, and British military officials went immediately to the lake, trying to figure out if this was some kind of weird enemy action. They determined that it wasn't, but it made news. When the summer came, and unusual heat made so much of the lake melt that corpses, some of which still had fragments of their skin and hair, began floating up out of the water and rolling against the sides of the lake, the news became more sensational still. The lake was nicknamed Mystery Lake, or, by those who wanted to be more literal, Skeleton Lake.
Many people tried to figure out how that many people were killed and, apparently, dumped in the lake, but it was only in the 2000s that we understood what happened. Radiocarbon dating showed that the skeletons dated back to between 830 and 850 AD. Analysis of the skeletons showed that there were two major familial or ethnic groups. Further study showed that they were all killed by heavy blows to the skull. At first people thought that they were two warring groups who killed each other, but a closer look at the skulls showed something far more horrifying.
They were pilgrims. Most likely, they were undertaking a pilgrimage between holy sites. The primary travelers were all from the same area, and they had hired a group of porters who knew the area to carry their baggage through the high mountains. One day, they came to a lake, and probably seeing an opportunity to get some fresh water, carefully climbed down the slopes toward the water. They were most likely in the basin when clouds moved overhead. There's no way to tell who was the first person to be struck down. Probably many dropped at once. Possibly, though, some people lived long enough to realize that they were being hit by hailstones the size of tennis balls hurtling down from the sky, and that the long, smooth, bare slopes down to the lake left nowhere to run to take shelter.
Rains of giant hailstones have been reported into modern times, and do occasionally kill people, but they rarely last long. Mystery Lake showed what happens if they hit at exactly the wrong time. It's possible that a few people might have hid under luggage or other people's bodies and made it out of the ravine after the storm was over. It's also possible that the entire party was pinned down and were all essentially stoned to death in a manner of minutes. The long slopes of the hills are still covered with loose gravel and stones. The bodies must have been slowly pushed down the sides of the ravine by landslides or rain, or possibly the hail itself. The waters of the lake were so cold, that they bodies were mostly preserved for a thousand years, waiting beneath the surface.