Cave of the Crystals, Crystal cave in Naica, Mexico.

Cave of the Crystals is a horseshoe-shaped limestone chamber in the Naica mine, about 109 metres long, whose floor is covered with faceted crystalline blocks. Huge selenite beams jut from the floor and blocks, crossing and leaning against one another; the largest measures 11.4 metres and an estimated 12 tonnes.

The two miners Eloy and Javier Delgado came upon the chamber in 2000 while drilling as they pushed deeper into the mine. The crystals had formed during the past 500,000 years as mineral-rich water interacted with limestone.

The name refers to the transparent gypsum crystals that glow like frozen light when mine lamps illuminate them. Scientists from different countries conduct field studies here and collect samples for laboratories investigating crystal growth under extreme conditions.

The chamber sits about 290 meters (950 feet) below the surface and maintains a constant 58 degrees Celsius (136 degrees Fahrenheit) with 90 percent humidity. Even with protective gear people can only spend about 10 minutes inside before they need to retreat.

The selenite beams here are the largest naturally grown crystal structures discovered anywhere on Earth. Researchers have found ancient microorganisms possibly more than 50,000 years old inside tiny fluid pockets trapped within the crystals.