Naica Mine entrance. Naica Mine is a silver, zinc and lead mine in Chihuahua, Mexico, that contains chambers filled with the largest known crystals on Earth. The crystals are formed of the selenite form of gypsum (calcium sulphate) and 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. There are two main crystal caves in the mine, the Cueva de las Espadas (Cave of Swords), 120 metres deep and discovered in 1912, and the Cueva de los Cristales (Cave of Crystals), 290 metres deep and discovered in 2000. The latter has the larger crystals, as its water cooled more slowly.