The Naica Mine of the Mexican state of Chihuahua, is a lead, zinc and silver mine that also contains extremely large selenitecrystals. Located in Naica in the municipality of Saucillo, the Naica Mine is owned by Industrias Peñoles, the world's largest silver producer. Caverns discovered during mining operations contain crystals of selenite as large as in diameter and long. Peñoles announced in October 2015 that it was indefinitely suspending operations due to uncontrollable flooding at the Naica Mine. The peak underground temperature is 58° C with 99% humidity, which interferes with breathing and increases the risk of asphyxiation. Because of the heat, unprotected researchers can only stay up to 10 min inside the cave. Longer visits require the use of a specialized suit with an oxygen supply.
Cave of the Crystals
The Cave of Crystals is a cave approximately below the surface in the limestone host rock of the mine. The chamber contains giant selenite crystals, some of the largest natural crystals ever found. The selenite crystals were formed by hydrothermal fluids emanating from the magmachambers below. The cavern was discovered while the miners were drilling through a newly drained area. The caves are closed to the public, and remain closed after a worker tried to enter the cave to steal some of the selenite, only to suffocate and die in the cave's humid and inhospitable atmosphere. Reports in 2017 stated that scientists had found "long-dormant microbes" in the crystals and removed them for further research. The cave was closed in 2015 and some of the chambers were allowed to flood again to continue the process of crystal growth. If the mining company decides to open another entrance, researchers might again enter to continue their work, according to a February 2019 report.
The Cave of Swords is another chamber in the Naica Mine, also containing gypsum crystals but each "only" about a meter long, due to the fact that these crystals are younger and had been growing for much less time by the time they were discovered in 1910.
