Cave of the Crystals or Giant Crystal Cave is a cave connected to the Naica Mine 300 metres (980 ft) below the surface.This is in Naica, south of Chihuahua, Northern Mexico. Naica mine is one of Mexico's most productive silver mine.
The main chamber contains giant selenite crystals (gypsum, CaSO4·2 H2O), some of the largest natural crystals ever found. The cave's largest crystal found to date is 12 m (39 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight.
The cave is extremely hot with air temperatures reaching up to 58 °C (136 °F). This is because about 5KM below the cave it is the molten lava in earths crust.
In order to counter the heat, one must wear specially designed refrigerated suite, packed with small ice cubes. Its not only the heat which is lethal but the humidity as well. Humidity is about 90 to 99 percent. With that amount of humidity moisture will get in to the lungs condensed. Therefore breathing apparatus is a must inside the cave. The cave is relatively unexplored due to these factors. Without proper protection people can only endure approximately ten minutes of exposure at a time.
Heat from the earth crust heated up the water and dissolving the minerals from the surrounding rocks. Heavy pressure forces this water to these caves. In the caves minerals were crystallized over a million years.
More Info at http://en.wikipedia.org/wiki/Cave_of_the_Crystals