Cave of Swords, at Naica Mine, Mexico. This cave is filled with large crystals of selenite, a form of gypsum (calcium sulphate). The cave lies around 120 metres below the surface. It is some 180 metres above the Cave of the Crystals, which contains even larger crystals. Both formed as an underlying magma chamber heated groundwater in and around the cave, which dissolved minerals to form a concentrated solution. The water was trapped in the cave at around 50 degrees for 500, 000 years, allowing the slow formation of the enormous crystals. This cave was discovered in 1910, when the cave was drained by the company operating the Naica Mine. Pumping operations keep it and the other caves clear of water to allow mining and research, but the crystals deteriorate in air. Pumping operations will stop eventually, flooding the caves again.