This underground cave, filled with gigantic selenite crystals—a variety of gypsum—is truly one of a kind. It’s also called the Cave of Crystals or the “Sistine Chapel of Crystals.” “This is the Sistine Chapel of Crystals,” said Juan Manuel García-Ruiz, a geologist with the Spanish National Research Council (CSIC) and the University of Granada, in a 2007 National Geographic interview. Since then, the nickname has stuck among scientists.
The cave sits 300 meters beneath the town of Naica in the Mexican state of Chihuahua. The town lies on an ancient fault line above a magma chamber, and for millions of years the cave filled with mineral-rich solutions heated to roughly 54–58 °C—ideal conditions for crystal growth. The resulting crystals are enormous, more like tree trunks than crystals: some reach 11 meters long and four meters wide. The largest weigh about 55 tons, making them the biggest crystals discovered on Earth.
What else is known about this cave? The Cave of Crystals is horseshoe-shaped and connects to the Naica mining complex, a lead, zinc, and silver mine that flooded about ten years ago. Miners discovered it by accident in 2000—the Sánchez brothers were digging a new ventilation tunnel when they opened a cavity in the limestone and found enormous, milky-white crystals jutting in every direction. Geologists later identified the crystals as selenite, a sulfate mineral that forms when salts dissolve in groundwater. Selenite is so soft it can be scratched with a fingernail, Live Science reported.
Over the years miners have found several crystal-filled caverns in Naica, including the Cave of Swords, whose walls are lined from floor to ceiling with dagger-like crystals. But the Cave of Crystals is by far the largest: it measures about 110 meters across and holds up to 6,000 cubic meters—more than twice the volume of an Olympic swimming pool.
The same conditions that grew the crystals are lethal to humans. That danger is why the cave picked up a fearsome reputation. With temperatures near 58 °C and humidity at 90–100%, the cave is deadly. Even with specialized gear a researcher can spend only about 20 minutes inside; without gear a visit tops out at roughly 10 minutes. Walking between the crystals is risky because the surfaces are extremely slippery. Still, scientists keep trying to study the cave. Biologists, in particular, think the fluids trapped in tiny pockets within the crystals could harbor ancient microorganisms of unknown species.
