Cave of the Crystals or Giant Crystal Cave is a cave 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-meter long with a volume of 5,000 to 6,000 cubic meters.
The cave was discovered in April 2000 by brothers Juan and Pedro Sanchez while drilling in the mine. As of October 2015, the mine had reflooded and the cavern filled once more with the water rich in minerals required for the crystals to grow.
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 tones. When it was accessible, the cave was extremely hot, with air temperatures reaching up to 58 °C with 90 to 99 percent humidity. The cave is relatively unexplored because of these factors. Without proper protection, people can only endure approximately ten minutes of exposure at a time.
The Cave of Crystals is a horseshoe-shaped cavity in limestone. Its floor is covered with perfectly faceted crystalline blocks. Huge crystal beams jut out from both the blocks and the floor. The crystals deteriorate in air, so the Naica Project attempted to visually document the crystals before they deteriorated further.
According to wikipedia