miners searching for fresh ore deposits in 2000 came across an unexpected and awesome sight. Massive, milky-white crystals towered around them, filling a horseshoe-shaped cave. Luminous beams of gypsum bigger than telephone poles, nearly 12 m long and 1 m wide, gleamed in the miners’ lights, jutting in all directions out of the brown limestone walls, floors, and ceiling.
Peñoles carefully restricted access to the Cave of Crystals, to protect not only the crystals but also their visitors. Conditions within the cave push human physiology to the limits. The temperature is about 50 °C, with a relative humidity of over 90%: sweating has no cooling effect in that environment.
Because conditions are so harsh, researchers could stay in the cave for only 10–15 min at a time, Montero-Cabrera says. The cave is sealed off from the rest of the mine by two sets of doors, protecting the crystals from the outside environment and keeping the temperature and humidity in the antechamber tolerable for humans. Before each foray, a medical check was required to ensure that visitors were healthy enough to withstand the cave’s climate, she says.