The Cave of Crystals (Cuevo de los Cristales) was discovered in the Naica mine in Chihuahua, Mexico in 2000. Crystals in the cave are made of gypsum and measure 36 feet (11 meters) long and can weigh up to 55 tons—as long as school buses and as heavy as a small herd of elephants. The cave is located nearly 1,000 feet (300 meters) below the surface. The air temperature in the cave is 112°F (50°C) and the humidity is 90 to 100 percent. The crystals grew because the water temperature was 136°F (58°C)—the temperature at which the mineral anhydrite dissolves into gypsum. The crystallized form of gypsum is selenite. Conditions in the cave were constant, allowing the crystals to continue to form and grow to their large size until 1985, when miners used pumps to lower the water table, draining the cave. If the cave is allowed to fill with water again, the crystals will resume growing.