Amazing crystals in Mexico - Cueva de los Cristales

The BBC reports that the conditions that allow such giant crystals to form have been determined. The cave was "kept at 58 degrees for hundreds of thousands of years."

From the Naica Crystal Project homepage

Its temperature of 50º and humidity of 100 % could kill anyone ... requires special gear and techniques to allow researchers to remain in it for more than 8 minutes, after which, survival is currently impossible ...

See also the beautiful gallery of photos and the stunning video.