Nestling up to a cauldron of pressurized, molten rock is almost never a good idea. But in Mexico's Naica mine, the payoff is worth the risk.
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.