Editorial use only Cave of Crystals. Geologist in the Cave of Crystals (Cueva de los Cristales) in Naica Mine, Chihuahua, Mexico. The crystals are the largest known in the world, and are formed of the selenite form of gypsum (calcium sulphate). They formed over millions of years in the mineral-rich geothermally heated water that filled the caves. The crystals were discovered after the water was pumped out of the mine. The Cave of Crystals is 290 metres deep, and was discovered in 2000. Above it, 120 metres deep, is the Cave of Swords (Cueva de las Espadas), which was discovered in 1912. The crystals in this cave are smaller as its water cooled more rapidly.