In comic books and the movies, Superman had a crystal Fortress of Solitude. As it turns out, there is a giant crystal cave underground in Mexico, but so far no superheroes have been spotting using it as a clubhouse. Discovered in 1910 by a mining company, it was initially dubbed "The Cave of Swords" since it had thousands of gypsum crystals protruding from the walls. Tourism and exploration resulted in the destruction of the original cave, but another was discovered 90 years later, and it was even larger.
The cave sits on top of a fault line, which is on top of a layer of magma. These layers resulted in super-heated groundwater full of sulfite ions. When the groundwater was exposed to the oxygen in the cave, crystals began to form. It didn't happen overnight. Experts estimate that the crystals there today are close to a half a million years old. Temperatures in the cave are about 136 degrees Fahrenheit and researchers cannot spend more than 40 minutes in the cave, even with protective gear. When the mining company is done with the site, they will pump water back into the space, as that is how the crystals originally began growing, returning the cave to its natural state.