The Beautiful Crystal Cave That's Actually A Potential Death Trap

There are plenty of natural wonders in the world that are actually incredibly dangerous. The Blue Hole of Santa Rosa, for example, is an unassuming New Mexico lake that's actually a death trap, hiding a cave system that claimed the lives of two Oklahoma State University students in the 1970s. Likewise, in Wyoming there exists a terrifying secret below Yellowstone National Park. In fact, many of the most beautiful natural landscapes are some of the most hostile environments to exist on Earth, and a particularly stunning example is Mexico's Cave of the Crystals, an underground cavern filled with giant Selenite crystals. 

At first glance, the chamber seems like an otherworldly subterranean refuge, home to one of the most incredible examples of natural geology on the planet. For anyone who's seen the original Richard Donner "Superman" movie, the cave might even conjure memories of the titular hero's Fortress of Solitude, with giant translucent crystals jutting out from the cave walls, some of which are big enough to walk across.

But this is far from any sort of refuge, as behind its enchanting appearance, the Cave of the Crystals is nothing short of a death trap.

In 2000, miners working in Naica, Mexico came across an incredible discovery 951 feet under a mountain in the Chihuahuan Desert. The crew, who were mining for ore deposits, uncovered a vast cavern in the limestone rock of the mine, filled with some of the largest crystals ever discovered on the planet: giant selenite crystals that reach up to 36 feet long and weigh up to 55 tons. They are thought to have grown over hundreds of thousands of years and had been completely untouched throughout that time. As such, they are believed to be the purest form of selenite crystal in the world.

The Cave of the Crystals, or Cueva de los cristales, as it was dubbed, is a horseshoe-shaped chamber roughly 358 feet long with a volume of 180,000 to 210,000 cubic feet. But the multiple outcrops and giant columns of translucent selenite — a crystalized version of the mineral Gympsum — have transformed the cave into a science-fiction landscape, as alluring as it is mysterious. 

The discovery of the Cave of the Crystals followed the much earlier discovery of an entirely different cavern in the Naica mine. Back in 1910, miners uncovered what became known as the Cueva de Las Espadas, or "Cave of Swords", a chamber much closer to the surface that was similarly filled with selenite crystals. However, those crystals measured up to around 6.5 feet in length, which compared to their counterparts in the Cave of the Crystals is fairly small.

Ninety years after the Cave of Swords was uncovered, miners uncovered the Cave of the Crystals, with its comparatively gigantic Selenite crystals. But how did these incredible crystals form?

Crystals form in caves through a complex process that involves two key stages known as "nucleation" and "crystal growth." In the case of the Cave of Crystals, the giant selenite beams formed as a result of volcanic activity roughly 26 million years ago. Rising magma below the Chihuahua region created a mountain and flooded its caverns with mineral-laden water. Anhydrite deposits initially formed in the waters and, over thousands of years, those waters cooled and the anhydrite — a mineral that, unlike other minerals, does not contain water — began to dissolve. That is when the Selenite crystals began to nucleate, a process that involves molecules sticking together to form stable clusters.

As the anhydrite dissolved it kept the water supersaturated with calcium and sulfate, meaning the water contained more of the calcium and sulfate than it could dissolve. Supersaturation is a key stage of crystal growth as it provides more molecules to stick to the growing nucleus of the future crystal. Over what researchers estimate to be roughly a million years, this process gave rise to the giant Selenite crystals of the Cave of the Crystals.

Crucially, because of the depth of the cavern, the water cooled at a slower rate than in the Cave of Swords, which sits much closer to the surface. This allowed the water temperature in the Cave of the Crystals to remain in a transition zone between anhydrite and gypsum for a long period, thereby allowing the crystals to grow as large as they did.

After its discovery, the Cave of the Crystals had all its water pumped out, enough to create a small, artificial lake near the town of Naica. However, exploring the emptied chamber proved to be much trickier than simply venturing underground and wandering inside.

The cave is actually an extremely dangerous place for humans to be, with temperatures reaching a potentially deadly 136 degrees Fahrenheit. That makes the cave equivalent to Death Valley, the hottest place on Earth, which in 1913, hit a temperature of 134 degrees Fahrenheit. These lethal temperatures are a result of the Cave of the Crystals sitting much deeper than the Cave of Swords, at 951 feet below the surface. The Naica mine gets hotter with depth due to a large pool of magma that sits just about 2.5 miles below the cave itself, and which constantly radiates heat. It's this same pool of magma that allowed for the water to remain at the right temperature for the selenite crystals to form in the first place, but which today could prove fatal to any humans that venture into the cave and stay there too long.

As if the high temperatures in the Cave of the Crystals weren't enough, the humidity is equally, if not more, dangerous. The humidity level in the cave ranges between 90 and 99%. At these levels, it becomes incredibly dangerous for humans as it makes it almost impossible for us to sweat.

Sweating is our natural cooling mechanism and involves the release of water and salt through pores on our skin, which then evaporates into the air. The rate at which sweat evaporates from our skin depends on how much water is already in the air around us. Whereas dry air allows sweat to evaporate quickly, humid air full of moisture makes it much harder; at a 99% humidity level, it is virtually impossible for sweat to evaporate at all. When our natural cooling system is compromised in this way, it causes the human body to heat up and, in extreme cases, develop heat cramps or heat stroke, where the body essentially cooks itself and organ failure is a real risk.

In the Cave of the Crystals, such heat and humidity means human beings can only last about 10 minutes before having to leave. Speleologist and historian Carlos Lazcano was the first scientist allowed into the cave following its discovery, and in 2021 recalled the experience of entering the harsh environment alongside French caver Claude Chaber. Speaking to Mexico News Daily, he said, "We couldn't stay in it for more than five minutes at a go! If we tried to make it to six minutes, we felt like we were dying!" Now, 25 years after it was first discovered, the Cave of the Crystals remains only partially explored as a result of its unforgiving conditions.

If the searing temperature and deadly humidity of the Cave of the Crystals doesn't get you, there are plenty of other hazards that could prove fatal. In fact, the cave is somewhat of a danger to itself. Many of the giant selenite beams that crisscross the cavern are sturdy enough that visitors can walk along them. But doing so isn't the best idea as they are slick with condensation due to the extremely high humidity.

The sheer size of these selenite beams also puts them in a precarious state. The largest are thought to weigh 40–50 metric tons and after the cave was drained of the water that gave rise to the giant crystals in the first place, the beams sat unsupported. This increased the risk of cracks and structural damage, especially as gypsum is an especially soft mineral. In fact, the crystals in the Naica cave are so soft you could scratch them using your fingernail. Without the support of the water, then, these huge, soft crystals are in severe danger of collapsing.

The harsh conditions in the Cave of the Crystals haven't stopped scientists from venturing into the dangerous chamber to study its unique geology. As reported in Chemical and Engineering News, María Elena Montero-Cabrera, a researcher at the Center for Research in Advanced Materials in Chihuahua, recalled how researchers required a medical check before entering to ensure they were healthy enough to withstand the cave's humidity and temperature. Once inside, Montero-Cabrera said that members of the team could stay for only 10 to 15 minutes before having to leave.

This wasn't the first exploration of the cave, however. That happened back in 2006, when a team led by University of Bologna cave mineral specialist Paolo Forti used refrigerated suits to study the interior of the cave. These suits used tubing connected to a backpack filled with water and ice, which was then pumped through the tubing to keep the researchers cool. But even with those suits, the team was only able to last around 30 minutes at a time.

Still, all the expeditions into this alluring furnace of a cave have been worth it. Pollen experts, geochemists, geologists, hydrogeologists, and biologists have all been able to explore the rare natural wonder. In 2017, biologists even found ancient bacteria trapped in samples taken from the crystals. The microbes were thought to have been there for tens of thousands of years and were unlike anything ever discovered on Earth.

Fifteen years after the Cave of the Crystals was drained of the water in which the giant selenite crystals had grown, it was re-flooded after miners broke into an aquifer. While the groundwater didn't reach the Cave of the Crystals at first, it has since refilled the cavern, preventing any further exploration from taking place. 

Still, that doesn't mean the cave won't be drained again in the future, hopefully allowing for more exploration to take place. But then, submerging the remarkable selenite crystals in water is exactly what they needed. As Chemical and Engineering News reported in 2018, studies using fragments of a large crystal found that the selenite was susceptible to dehydration when left in air rather than water, which over time could "cloud and scar" the surfaces of the crystals. Thankfully, the cave has essentially been restored to the state it was in before discovery, which will help to protect the crystal formations, and prevent the giant columns of selenite from deteriorating any further.