Cave of the Crystals

Cave of the Crystals or Giant Crystal Cave (Spanish: Cueva de los Cristales) is a cave connected to the Naica Mine 300 metres (980 ft) below the surface in Naica, Chihuahua, Mexico. The main chamber contains giant selenite crystals(gypsum, CaSO₄·2 H₂O), some of the largest natural crystals ever found.^[1] The cave's largest crystal found to date is 12 m (39 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight. The cave is extremely hot with air temperatures reaching up to 58 °C (136 °F)^[2] with 90 to 99 percent humidity. The cave is relatively unexplored due to these factors.^[3] Without proper protection people can only endure approximately ten minutes of exposure at a time.^[4]

• http://en.wikipedia.org/wiki/Cave of the Crystals