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It looks like Superman's Fortress of Solitude and is nearly as hard to get into, but that hasn't stopped explorers from uncovering new secrets in and around Mexico's deep, deadly hot Cave of Crystals.

Outfitted with ice-cooled suits, teams have found biological mysteries, parallels with other planets, and the "Ice Palace," an unexplored cavern lined with rare crystal formations—and just in time too. Parts of the complex may soon be returned to their natural, submerged states.



A thousand feet (304 meter) underground, the Cave of Crystals (pictures) is just one of a series of glittering caverns beneath the Chihuahuan Desert's Naica mountain (map). Much of the complex would naturally be filled with scorching water, were it not for industrial pumps that facilitate the mining of silver, zinc, lead, and other minerals in the caves.

In the two-story-tall, football-field-size Cave of Crystals, enormous beams of gypsum—among the largest freestanding crystals in the world—sprout haphazardly from the ceiling, floor, and walls. Individually, though, the crystals appear anything but haphazard, sporting the sharp, geometric appearance that scientists call euhedral.

This jewel-like effect makes the giant crystals truly unique, according to John Rakovan, a mineralogist at Miami University in Ohio, who was not involved in the project.

"When crystals get larger and larger, they become less euhedral, typically"—and more rocklike. "Scientists didn't think it was possible to get large crystals that are so morphologically perfect" before the Cave of Crystals discovery, Rakovan said.

The translucent columns also resemble giant pillars of ice but are warmed by superheated air leaking up from underground magma chambers.

The combination of 90 percent humidity and a temperature of 118 degrees Fahrenheit (48 degrees Celsius) inside the cave can kill an unprepared human in just 30 minutes.

Beyond the Cave of Crystals

Discovered by miners in 2000, the Cave of Crystals is just one chamber in what appears to be a network of subterranean caverns beneath Naica—some well known and evocatively named: the Cave of Swords, the Queen's Eye, the Cave of Sails.

In 2009 a video camera attached to a drill bit found hints of one more crystal-lined cave during the creation of the Robin Hole, a 2,000-foot-deep (600-meter-deep) ventilation shaft meant to cool mining tunnels below.

Descending into the hole months later, in December 2009, a scientific team confirmed the new, naturally dry crystal cave about 500 feet (150 meters) below the surface.

Dubbed the Ice Palace, the new cave lacks giant pillars, but sparkles with rare crystal formations, including minerals resembling cauliflower and fiber-optic-like filaments.

The mysteries of the Ice Palace will likely remain unsolved, however. The Industrias Peñoles mining company has decided to cover the Robin Hole and has also hinted that it may shut down the expensive water pumps that keep the Cave of Crystals dry, according to the documentary.

"I don't think they'll ever be able to preserve those caves," Miami University's Rakovan said. "It'll be economically unfeasible."

But shutting down the caves isn't necessarily a bad thing, Rakovan added. "It might actually preserve the crystals. And if at a later date it becomes important to get in there again, they could repump."

Ice-Cooled Suits Required

Exploring the Naica caves requires more than just industrial strength water pumps. Scientists entering the complex in 2008 and 2009 wore custom-made, 45-pound (25-kilograms) cooling suits that extend mission times from 15 minutes to an hour.

Each suit contains several ice-filled compartments as well as respirators connected to ice-filled backpacks, which send cool air to the wearer's lungs. Masks protect the eyes, which can scorch in the cave's heat.

"It's funny, because when you look at the pictures of us in there in the suits, it looks like we're in an ice chamber, but it's just the reverse," said Boston, of the New Mexico Institute of Mining and Technology.

Even with the protective gear, the scientists often operated at the edge of safety.

Ontario, Canada-based astrophysicist Sara Poirier said, "We all just kept pushing further and further [into the cave], so by the time you are heading back, you've pushed yourself beyond the limits that are safe"—and that's with the gear operating properly.

At least one piece of equipment offered little protection. The portable Ice Cube—a newly designed, air-cooled, transparent tent designed to allow for two-hour Naica-caves missions—never cooled down enough to bring overheated bodies back to safe levels.

Microbial Mystery in Cave of Crystals

Though the calling card of the horseshoe-shaped Cave of Crystals may be its massive mineral formations, some of its biggest surprises are literally microscopic.

In 2008 a team of scientists, including New Mexico Tech's Boston, investigated the cave and found microbial life living in tiny air pockets in the crystals.

In December 2009 Boston returned to the cave with another team. From pools of water that hadn't been present during her first trip, the scientists collected bacteria as well as viruses that prey on the bacteria—something that was suspected but had not been confirmed on the first expedition.

Viruses, after all, are among the "primary predators of bacteria," explained Danielle Winget, a biologist at the University of British Columbia, in the new documentary.

Sure enough, the team found as many as 200 million viruses in a single drop of Cave of Crystals water.

But the virus finding was perhaps not the expedition's most surprising microbial discovery. Analysis of bacterial DNA from the Cave of Crystals showed that the tiny life-forms are related to microbes living in other extreme environments around the world, including caves in South Africa and Australia as well as hydrothermal vents (video).

"We're picking up these patterns of similarities in places that are geographically widely separated," Boston said.

That similarity and separation adds up to a mystery, according to Curtis Suttle, a biologist at University of British Columbia and a member of the 2009 Cave of Crystals expedition.

"We don't really understand how it is that the organisms in a hydrothermal vent in Greece or a deep gold mine in South Africa are related to organisms that we find in a subsurface cave" at Naica, Suttle said.

"It's hard to imagine some kind of underground [network] connecting South Africa with Mexico."

Alien Underworlds

As mind-boggling as the idea of a possibly globe-spanning, underground bacterial network may be, some scientists see potential links between the Cave of Crystals and even farther-flung hot spots—for example, extreme environments on Mars and other worlds.

Though Martian geology might be more static overall than Earth's, "there may be residual pockets of geothermal activity that could provide a zone where water could be liquid and where chemically reduced gases from below can percolate up and act as a nutrient source," as in the Cave of Crystals, Boston said.

Poirier, the Ontario astrophysicist, agreed.

"For Mars, our best bet of finding life is to look underground," Poirier said. "So there are a lot of parallels between humans exploring subterranean caves looking for microbes and Martian exploration in the future."

If the caves on Mars are anything like the caverns beneath Naica mountain, she said, future Martian explorers will have to be trained to ignore the strange sights surrounding them.

"When you're in the caves, you're overwhelmed by the [harsh] conditions, but you're also overwhelmed by the beauty, and it's really hard to maintain your focus," she said.

Even if scalding water submerges that beauty tomorrow, Boston said, the caves' scientific potential should live on, thanks to the multitude of samples already collected.