In 1942 a British forest guard in Roopkund, India made an alarming discovery. Some 16,000 feet above sea level, at the bottom of a small valley, was a frozen lake absolutely full of skeletons. That summer, ice melt revealed even more skeletal remains, floating in the water and lying haphazardly around the lake's edges. Something horrible had happened here.
A National Geographic team set out to examine the bones in 2004. Besides dating the remains to around 850 AD, the team realized that everyone at the "Skeleton Lake" had died from blows to the head and shoulders caused by "blunt, round objects about the size of cricket balls."
This eventually led the team to one conclusion: In 850 AD this group of 200 some travelers was crossing this valley when they were caught in a sudden and severe hailstorm.
Arlas Obscura has more stories of killer hailstorms from ancient times to the 21st century. Link -Thanks, Dylan!