Giant selenite crystal. Geologist's hand touching a selenite crystal from the Cave of Crystals (Cueva de los Cristales) in Naica Mine, Chihuahua, Mexico. This cave contains the largest natural crystals ever found on Earth. The crystals formed over millions of years by growing in the mineral- rich geothermally heated water that filled the cave. The cave is 290 metres below the surface, and the crystals were discovered in 2000, after the water in the mine was pumped out. A higher cave, the 120 metre deep Cave of Swords (Cueva de las Espadas), was discovered in 1912. Its crystals are smaller as the water in that cave cooled more rapidly.