Currently, the high temperature is 58° C (136° F) with 99% humidity, which interferes with breathing and increases the risk of asphyxiation. Because of this, researchers require specialized suits with an oxygen supply to survive working in the environment.
The Cave of Crystals is a cave approximately 300 m (1,000 ft) 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 magma chambers 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. After enough research on the crystals has been carried out, some of the chambers will be allowed to flood again and continue the process of crystal growth.
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.