Exploring Mexico's Sacred Caves

Some cenotes are more than 100 meters (328 feet) deep. Normal compressed air is no longer sufficient at these depths. To avoid nitrogen narcosis and oxygen poisoning, cave divers use other products, including Trimix, a mixture of oxygen, nitrogen and helium. If they encounter problems despite the many precautions, rapid ascents are not possible because divers have nothing but rock above their heads. "Sometimes we penetrate more than a kilometer into a cave," says Uli Kunz, an underwater photographer and biologist. "If, under these circumstances, you calculate your air supply incorrectly, get lost or panic, you're as good as dead."

The Kiel-based dive team, which has been working in Mexico's caves since 2009, started using modern imaging technology in 2010. Some 63 square meters of the Las Calaveras cenote were recorded using photogrammetry, and wraparound photo series of individual relics were processed into 3-D models on the computer. "This is a powerful tool for research," says Guillermo de Anda of the Autonomous University of Yucatan, the leading expert on Mexican cenotes. Of course, he says, some finds still have to be recovered, such as when scientists want to perform genetic analyses of chemical dating procedures. "But a lot can be discovered with 3-D models, without even touching or possibly destroying the find," he says.

The method also enables archeologist to evaluate relics without having to dive into cenotes. Likewise, it offers benefits to the few experts on antiquity who are also research and cave divers. "Underwater, you can usually spend only a few minutes with the finds," says de Anda. "But, on the computer, you have all the time in the world."

Other high-tech methods are also suited to exploring the world of caves. Laser scans taken from the air, for example, make it possible to eliminate entire forests on the computer. Scientists have used this method to find overgrown Mayan infrastructures, which in turn point to cenotes.

How to Shrink a 3-D Film Camera

Making the planned documentary is even more complex than preparing computer models of individual relics. Previous models of 3-D film cameras, including their underwater housings, were almost as tall as a man and weighed hundreds of pounds. Using them in the narrow caves would have been unthinkable. Specialist companies from Hamburg and Kiel designed and created a camera housing that was small and lightweight enough. "It took half a year just to develop and build the housing," says Peter Baaten, the film's producer.

But the uniqueness of the filming location will likely justify the expense. Some cenotes are so beautiful that they seem almost romantically staged, such as when thick beams of sunlight penetrate the entry hole and bathe the underground world in shimmering light. The water is often so clear that divers feel like they're floating through air.

Other cenotes could double as sets for horror films. In the Angelita cenote, a giant, circular hole, there is a 30-meter layer of fresh water on top of the saltwater -- as in most cenotes. Between the two is a billowing sulfate layer that looks like a blanket of muddy clouds. The branches of giant trees, submerged long ago, protrude into the twilight like dead fingers. Scenes like this help to explain why the Mayans felt that the cenotes were gates into hell.

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