The largest known natural crystals in the world are in the Crystal Cave of the Giants, within Naica mine in northern Mexico. The cave is filled with selenite crystals as long as 36 ft (11 m) and weighing up to 55 tons. These crystals are so impressive that when pictures of them first hit the Internet, the vast majority of observers labeled them as fake. These natural crystals achieved their size by growing for millions of years in near-perfect conditions, submerged in mineral-rich water with a consistent and optimal temperature of 136 degrees F (58 degrees C).
The cave of natural crystals was discovered in 2000, when miners Juan and Pedro Sanchez were excavating a new tunnel in Naica mine for the Industrias Penoles company. Earlier drilling had emptied the lower reaches of the mine of water, and the miners were looking for mineral deposits in unexplored regions of the mountain. What they found was unprecedented. The engineer in charge, Roberto Gonzalez, immediately recognized the significance of the find and ordered the tunnel to be rerouted. The chamber had a temperature of 150 degrees F and had 100% humidity. Without protective gear, miners could only spend about eight minutes there.
Unfortunately, in the early weeks after the natural crystals were uncovered, there was damage to them as unscrupulous miners tried to carry some of them off. One miner that snuck into the cave to steal crystals brought plastic bags full of air in an attempt to lengthen the amount of time he could spend there; unfortunately for him, this strategy (predictably) failed. He passed out and was later found thoroughly baked. Soon after, the mining company installed an iron door to protect the marvelous natural crystals. Unfortunately, Mexico's laws do not protect minerals from theft, meaning that only a physical barrier can stop thieves.
The mining company that discovered the Crystal Cave of the Giants would like to develop the cave for tourism, but doing so could be difficult. At about 1000 ft (304 m) beneath the surface, the cave is deep for a public visit, plus something would have to be done about the high temperature. There is the worry that lowering the temperature to a more tolerable range might damage the crystals. To protect the cave, the idea of applying for World Heritage Site status has been floated. It may be that the cave is never opened to the public.