If you visited Spain in January 2000, you needed a tin umbrella: Chunks of ice weighing up to 6.6 pounds fell out of a cloudless sky for 10 days.

No one knows how the ice formed. The chunks resembled hailstones, but no thunderstorm was present.

Planetary geologist Jesus Martinez-Frias dubbed the stones megacryometeors, and more than 50 have been recorded since the Spanish fall. The largest, in Brazil, weighed 485 pounds.