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) with 90 to 99 percent humidity. The cave is relatively unexplored due to these factors. Without proper protection people can only endure approximately ten minutes of exposure at a time. A group of scientists known as the Naica Project have been heavily involved in researching these caverns. Naica lies on an ancient fault and there is an underground magma chamber below the cave. The magma heated the ground water and it became saturated with minerals, including large quantities of gypsum. The hollow space of the cave was filled with this mineral-rich hot water and remained filled for about 500,000 years. During this time, the temperature of the water remained very stable at over 50 °C (122 °F). This allowed crystals to form and grow to immense sizes. In 1910 miners discovered a cavern beneath the Naica mine workings, the Cave of Swords (Spanish: Cueva de las Espadas). It is located at a depth of 120 m, above the Cave of Crystals, and contains spectacular, smaller (1 m long) crystals. It is speculated that at this level, transition temperatures may have fallen much more rapidly, leading to an end in the growth of the crystals. The Giant Crystal cave was discovered in 2000 by miners excavating a new tunnel for the Industrias Peñoles mining company located in Naica, Mexico, while drilling through the Naica fault, which they were concerned would flood the mine. The mining complex in Naica contains substantial deposits of silver, zinc and lead. 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 caves are accessible today because the mining company's pumping operations keep them clear of water. If the pumping were stopped, the caves would again be submerged. The crystals deteriorate in air, so the Naica Project is attempting to visually document the crystals before they deteriorate further. The cave was featured on the Discovery Channel program Naica: Beyond The Crystal Cave in February 2011. Exploration hinted at the existence of further chambers, but further exploration would have required demolition of the crystals. It was stated that the cave would eventually be resealed and the water level allowed to rise again.