Hand touching one of the huge selenite crystals from the Cave of the Crystals, Naica Mine, Mexico. This cave is filled with enormous crystals of selenite, a form of gypsum (calcium sulphate). The cave lies around 300 metres below the surface and is extremely hot, with temperatures reaching nearly 60 degrees Celsius. It 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. The cave was discovered in 2000, when the cave was drained by the company operating the Naica Mine. Pumping operations keep it clear of water to allow mining and research, but the crystals deteriorate in air. Pumping operations will stop eventually, flooding the caves again.