About 900 feet below the surface, there is a chamber filled with gypsum. It's the same stuff that goes in the drywall in your house, only in Naica it spent half a million years parboiling in a chamber filled with magma-heated water.
Suddenly miners showed up and started pumping the mineral-rich broth out to get at valuable silver and lead deposits nearby. The result is a cavern filled with crystals 36 feet long and weighing in at up to 55 tons, easily the largest in the world.
"When we first arrived at the Naica mine, Manuel and his crew took us inside without wearing the special cooling suits. This was in order to get us used to what REAL heat is like. There is a steel door protecting the cave and as soon as you pass through it, the temperature hits you like a truck.
But as soon as you get your first glimpse of the incredible crystals, you want to keep going deeper. We were inside for only 14 minutes, which was pushing the danger limits without cooling suits. When we exited, the staging area was a "cool" 41 Celsius. My heart was pounding and I was completely soaked in sweat, my shirts, pants, socks & boots... Everything. All we could do was sit, drink and rest."
http://news.discovery.com/earth/naica-big-pics.html