About fifteen years ago, when a mining company started pumping water out of a cave 300 meters below Naica, Mexico, they were hoping to find lead or  zinc or maybe some silver.    Once they finished pumping the water out, though, they realized that they weren’t going to be mining anything from this cave – mostly because of the giant, twelve-meter-long spikes everywhere.   The spikes were actually huge crystals, the largest ever found.   And the pillars of mineral -- as big as three story buildings -- weren’t the only hazard in the cave. If you managed to make the same descent the miners did, you’d only get to see the crystals for about ten minutes, because that’s roughly how long a human can survive unprotected in the cave.   The appropriately-named Giant Crystal Cave was discovered in 2000, when miners were drilling a tunnel to expand their mine. They’d already pumped the water out of the area, and as they blasted through the rock, they revealed the thirty-by-ten meter cave, containing enormous crystals of selenite -- a crystal form of the mineral gypsum.   The conditions in the caves are a collection of extremes. The thermometer reads about 58 degrees Celsius on a good day, and the humidity hovers around 100 percent. The cave contains hundreds of crystals, some up to twelve meters long and weighing about 50 metric tons.    It’s so hot and humid in the Giant Crystal Cave that the insides of your lungs would actually be the coolest surface around. So the water vapor in the air would condense in your lungs as you breathed, and that is not a great thing to have happen to  you.   Essentially, if you stayed in the cave long enough, you’d drown in your own lungs.   So, understandably, these conditions make scientific research kinda tough.    Scientists have to go in there with custom-made, super-powerful refrigeration suits, but even those only buy them about half an hour at a time.    So we still have a lot to learn about the cave and what’s in it -- maybe even forms of life we've never seen before.   It might seem unlucky that the cave with the largest crystals in the world is so inhospitable, but those conditions are actually the perfect nursery for these mineral formations.   The cave lies deep beneath Naica mountain, which formed about 26 million years ago from volcanic activity. And all of that molten lava came with a bonus: a whole lot of anhydrite.   Anhydrite is the dehydrated form of the mineral gypsum -- both compounds have the same chemical makeup, except for the anhydrite is missing water.    But if conditions are right, anhydrite can absorb water and become gypsum.    And if the anhydrite turns into gypsum slowly enough, it will crystallize, to form selenite -- just one of the many forms that gypsum can take.     The real key to this process is temperature. If it's above 58 degrees, anhydrite will be stable, and just sit there. But at or below 58 degrees, it’ll begin to dissolve, and use the water in the environment to essentially re-form into gypsum.   And that is exactly what happened in Giant Crystal Cave.   Once the magma beneath the mountain cooled to around 58 degrees, the anhydrite began dissolving very, very slowly. Then, new particles of the hydrated gypsum began to form as crystals of selenite.    Now, other caves in this same mountain -- like the one with the particularly boss name of Cave of Swords -- also contain lots of selenite crystals.    But the crystals there are only about a meter long, because that cave cooled more rapidly, so the hydrated gypsum formed too quickly to deposit as very large crystals.   But Giant Crystal Cave was just deep enough in the ground to stay at the transition temperature. Since the cave was around 58 degrees for about a half a million years and filled with anhydrite-rich water, the selenite crystals just kept on growing until the year 2000, when we pumped the water out.   Technically, there’s no limit to how big the crystals can get in such perfect conditions. So, when the mine eventually closes and we stop pumping the water out of the cave, the crystals will again start growing.   Thanks for watching this SciShow Dose! If you have an idea for a Weird Place that you’d like to explore and share with the world, let us know in the comments below, or on Facebook or Twitter or Tumblr.   And don’t forget to go to youtube.com/scishow and subscribe!