Description of exploration of the Crystal Cave of Giants. The explorer, George Kourounis, has other photo galleries as well.

From the site:

… The crystals themselves are made of selenite which is crystallized gypsum, the same material used in drywall construction. Except these crystals formed over a span of about half a million years in a hot water solution, saturated with minerals. The the temperature inside the cave remained very consistently hot for the entire time the crystals were growing.…