Naica Cave of Crystals or Giant Crystal Cave is a U-shaped cavity in the limestone below the Sierra de Naica connected to the Naica Mine at a depth of 300 meters, in Naica, Chihuahua, Mexico. It takes the form of a chamber within the limestone host rock of the mine and is about 109 meters long with a volume of 5,000 to 6,000 cubic meters.
Giant Crystal Cave itself wasn’t found until the year 2000. The water was pumped out by the Peñoles Mining Company before anyone realized that it was full of gargantuan selenite columns. Brothers Pedro and Juan Sanchez — two miners with the organization — became the first people to lay eyes on its crystals when they entered the drying cave on foot.
The chamber contains giant selenite crystals (gypsum, CaSO4 • 2 H2O), some of the largest natural crystals ever found. The largest is 11.40 meters, with a volume of about 5 cubic meters, and an estimated mass of 12 tons. When not flooded, the cave is extremely hot, with air temperatures reaching up to 58 °C (136 °F) with 90 to 99 percent humidity.
This is comparable to temperature records in Death Valley, but with much wetter air. The cave is relatively unexplored because of these factors. Without proper protection, people can only endure approximately ten minutes of exposure at a time.
Longer visits required the use of a special suit fitted with a cooling system (ice reserve in a backpack with cooled water recirculating in polymer tubes installed over all the body surface) and delivering fresh air for easier breathing. The visits were limited to about half an hour, the time needed to melt the ice reserves.
According to the Internet