Deep beneath a silver, lead and zinc mine in Mexico’s northern Chihuahuan Desert lies a chamber that looks like a set from a science fiction film.
Translucent beams of crystal, as thick as tree trunks and as long as a city bus, crisscross the cavern in every direction. This is the Naica Cave of the Crystals, a place of staggering beauty and lethal hostility, where no one could last more than a few minutes without protection.
An Accidental Discovery
Naica sits in the municipality of Saucillo, about 80 miles south of Chihuahua City. For decades, the mining company Peñoles operated one of Mexico’s most important mines here. To keep the tunnels dry, it pumped hot groundwater out of the earth around the clock.
In April 2000, brothers Eloy and Javier Delgado were drilling a tunnel about 980 feet underground when they broke through into a chamber. Peering inside, they saw giant crystals. The cave had been flooded for hundreds of thousands of years, and the mine’s pumping had lowered the water table just enough to expose it.
The main chamber is about 360 feet long. The largest crystals reach about 36 feet in length and measure around 13 feet across. Estimates of their weight range from 13 to 61 tons. That makes them the largest natural crystals ever found.
The Cave of the Crystals is the most famous of Naica’s chambers, but it is not the only one. The mine holds three other crystal-lined caves: the Cave of Swords (discovered in 1910), the Cave of Candles and the Queen’s Eye Cave. None, however, rivals its spectacle.
How They Formed
The Naica crystals are made of selenite, the transparent form of gypsum.
To understand why they grew so large, you have to look deeper still. Beneath the Sierra de Naica lies a magma chamber that, for hundreds of thousands of years, kept the groundwater hot. That water was rich in calcium sulfate.
As long as the water stayed above about 136 degrees Fahrenheit, the dissolved mineral took a different form: anhydrite. Over time, though, the water cooled. Once it dipped below that threshold, the anhydrite started to dissolve, and the calcium sulfate slowly crystallized as gypsum.
The key was stability. The temperature held nearly constant for an extraordinarily long time, allowing the crystals to grow at an almost imperceptible pace, without interruption. Calculations suggest the process lasted at least 500,000 years.
Crystallographer Juan Manuel García-Ruiz of the University of Granada pieced this together by studying tiny droplets of fluid trapped inside the crystals, which preserve a record of the water they grew in. It was García-Ruiz who dubbed the cave “the Sistine Chapel of crystals.”
A Place Nearly Impossible to Visit
Studying the cave was anything but easy. Temperatures reached about 136 degrees, with humidity of 90% to 99%.
Astrobiologist Penelope Boston, one of the scientists who ventured inside, said no one could endure more than five minutes at a stretch without protection.
Researchers descended in specialized suits, with ice packs strapped to their bodies and respirators that cooled the air they breathed. Even so, they could work for only about 20 minutes before retreating to an air-conditioned rest chamber.
The Naica Project, the research effort behind these expeditions, advised against staying inside for more than half an hour.
Life Trapped in Crystal
The crystals held one more surprise.
In 2017, Boston, then director of NASA’s Astrobiology Institute, announced that her team had found dormant microbes inside the fluid pockets sealed within the crystals. By her estimate, they had been trapped there for 10,000 to 50,000 years. The researchers later revived them in the lab.
The organisms fed on minerals such as iron and manganese, and their DNA was strikingly different from that of any known species. Other scientists urged caution, questioning in particular whether anything could survive that long.
Still, the discovery intrigued astrobiologists, because it hints at the kind of life that might exist in extreme environments, such as beneath the surface of Mars.
Can You Visit?
No. The cave was never open to tourists, and today it remains sealed off.
In 2015, the mine stopped pumping water, and the chamber flooded once more. The crystals now lie submerged in the same hot water in which they grew.
For now, the only way to see them is through photographs, documentaries and scientific papers. If you want to see giant crystals in person, try the Pulpí Geode in Almería, Spain, a much smaller cave open for guided tours.
What’s most striking about Naica is how late we arrived. For half a million years, these crystals grew in darkness, in scalding water, with no one to see them. When miners finally broke through, the heat made sure no one could stay.
Perhaps that is why they still carry an aura of mystery. Naica revealed its beauty for a few brief years, then slipped back into hiding.
