The Naica Crystal Caves in Mexico contain the largest natural crystals ever discovered. Some of the clear selenite crystals reach over 50 feet in length and weigh over 55 tons. The caves have extremely high temperatures of over 150 degrees Fahrenheit and 100% humidity, allowing humans to remain for only 6-10 minutes before risks to health. The mining company plans to air condition the caves and develop them as a tourist attraction to preserve the rare crystal formations.