Here’s a really wild video of the Naica Cave in Chihuahua, Mexico. More than 300 meters below the Earth’s surface, super-heated water separated minerals as the water was forced through the above layers of rock. As the water reached a pocket of earth far enough from the lava to cool but close enough to the earth’s core to remain pressurized and heated, the minerals reformed as the crystals you see in the video.
You can find more information about the cave and watch a trailer for a documentary about the cave at the NAICA la Pelicula website.
