The Cave of the Crystals in Naica is home to some of the largest natural crystals ever found. These selenite (gypsum) crystals can reach up to 12 meters in length and weigh 55 tons. The cave was discovered in 2000 by miners. The crystals grew to such enormous sizes because they were submerged in mineral-rich water at a stable temperature of about 58°C (136°F) for hundreds of thousands of years. Because the cave is so hot and humid (90% to 99% humidity), humans can only stay inside for a few minutes without specialized cooling suits. Since mining operations ceased in 2015 and the pumps were turned off, the cave has refilled with water, which will allow the crystals to continue their slow growth undisturbed by human activity.