Cave of Swords. Selenite crystals in the Cave of Swords (Cueva de las Espadas) in Naica Mine, Chihuahua, Mexico. The crystals 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 Swords is 120 metres deep, and was discovered in 1912. A still-deeper cave, the 290 metre deep Cave of Crystals (Cueva de los Cristales), was discovered in 2000. The crystals in that cave are the largest known in the world. They are larger than those in the Cave of Swords as the water in the Cave of Crystals cooled more slowly.