The Naica Mine in Chihuahua, Mexico is a lead, zinc, and silver mine that contains huge selenite (gypsum) crystals. The largest crystals are 4 feet in diameter and 50 feet long. Depicted here, you can see the scale against the miner in the lower right corner. The caves of the mine sit above magma chambers which are thought to pushed hydrothermal fluids up to create the selenite crystals. Because of the magma the cave is inhospitably hot. The peak underground temperature is 136° F (58° C) with 99% humidity. This environment makes it impossible for unprotected researchers to spend more than 10 minutes in the caverns. In 2015 the mines were closed and some chambers allowed to flood to continue crystal growth. Other minerals found in the mines include galena, sphalerite, calcite, and fluorite. Image by By Alexander Van Driessche - Gaianauta received this from Alexander Van Driessche via Email., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=23231964. Information from Wikipedia.com.