Cave of Crystals, 2002, Chihuahua, Mexico:

Largest crystal cave in the world was uncovered! Faceted Selenite (Gypsum) crystals were found 1000 feet underground, some measuring 36' tall and 55 tons -- EACH.

Due to extreme heat and humidity (and at least one death), however, the mine is now closed to the public. Check out the National Geographic Channel for reruns of this spectacular discovery!