Cave of the Crystals is connected to the Naica Mine 980 ft. below the surface in Naica, Mexico. The main chamber contains giant gypsum crystals, some of the largest natural crystals ever found. The largest crystal found to date is 39 ft. long, 13 ft. in diameter and weighs 55 tons. The cave is extremely hot, with air temperatures reaching up to 136 °F with 90 - 99 percent humidity. Without proper protection, people can only endure about 10 min. of exposure at a time.