980 ft (300 m) beneath the surface of Naica, Chihuahua, Mexico is a mine filled with giant gypsum crystals. Known locally as Cueva de los Cristales (Cave of the Crystals), the gypsum crystal cave is 98 ft (27 m in length) and connected to a larger, working mine best known for its selenite crystals.

According to National Geographic and BBC:

 

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) with 90 to 99 percent humidity. The cave is relatively unexplored due to these factors. Without proper protection, people can only endure approximately ten minutes of exposure at a time.

 

If you would like to learn more, be sure to check out the article on National Geographic entitled, Cavern of Crystal Giants. You can also check out these supplementary articles on National Geographic and BBC.