Crystal Cave of the Giants - another site, a much better explanation
These magnificent giants are made of natural minerals (duh) - selenite, the gypsum crystal,. The largest are 30 to 50 feet high and 3 to 4 feet in diameter. The temperature (150 degrees F)and humidity (100%) in the cave are such that a human can survive only 6 to 10 minutes before total mental befuddlement.
It's connected to a working mine in southern Chihuahua, Mexico. The caves have been pumped and drained but when the mining operations are done, the pumping will cease and the crystals will begin to grow again.
The pictures on the PPS were much more impressive, but again, it's the whole virus issue. Do a quick search on Google and you'll see plenty of links and images.
And SHOW THE KIDS!!