Originally Posted by wikipedia

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) deep in Chihuahua, Mexico. The main chamber contains giant selenite crystals, some of the largest natural crystals ever found.[1] The cave's largest crystal found to date is 11 m (36 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight. The cave is 27 m (89 ft) in length and 9 m (30 ft) in width. The cave is extremely hot with air temperatures reaching up to 58 °C (136 °F)[2] with 90 to 100 percent humidity. The cave is relatively unexplored due to the extreme temperatures and high humidity.[3] Without proper protection people can only endure approximately ten minutes of exposure at a time.[4]

A group of scientists known as the Naica Project have been heavily involved in researching these caverns.[5]