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Offshore flooded caves, so-called ocean blue holes, are extensions of the sea, subject to the same heavy tides and host to many of the same species found in the surrounding waters. Inland blue holes, however, are unlike any other environment on Earth, thanks largely to their geology and water chemistry. In these flooded caves, such as Stargate on Andros Island, the reduced tidal flow results in a sharp stratification of water chemistry. A thin lens of fresh water—supplied by rainfall—lies atop a denser layer of salt water. The freshwater lens acts as a lid, isolating the salt water from atmospheric oxygen and inhibiting bacteria from causing organic matter to decay. Bacteria in the zone just below the fresh water survive by exploiting sulfate (one of the salts in the water), generating hydrogen sulfide as a by-product. Known on land as swamp or sewer gas, hydrogen sulfide in higher doses can cause delirium and death.

As living laboratories, inland blue holes are the scientific equivalent of Tut's tomb. From a diver's perspective, they're on par with Everest or K2, requiring highly specialized training, equipment, and experience. Even more than high-altitude mountaineers, cave divers work under tremendous time pressure. When something goes wrong, if they don't solve the problem and make it back to the cave entrance before their gas runs out, they're doomed.

Until now, only a handful of scientists have ventured into blue holes, but in the summer and fall of 2009, a multidisciplinary cave-diving and scientific team spent two months studying them on Andros, Abaco, and five other Bahamian islands. Funded by the National Geographic Society in collaboration with the National Museum of the Bahamas, headed by Keith Tinker, the Bahamas Blue Hole Expedition was conceived by Kenny Broad, a veteran cave explorer and an anthropologist at the University of Miami. Under Broad's wisecracking, driven leadership, with Brian Kakuk as dive safety officer and preeminent cave explorer Wes Skiles shooting film and stills, team members made around 150 dives in dozens of blue holes. They gathered data that promise to deepen our understanding of everything from geology and water chemistry to biology, paleontology, archaeology, and even astrobiology—the study of life in the universe.

They worked with urgency. At the current rate of sea-level rise (possibly several feet over the next century), many inland caves will flood with seawater in the coming decades, disrupting their delicate chemistry and destroying the very conditions that make them so valuable to science. Meanwhile, blue holes are often used as dumping grounds, polluting the islands' primary source of natural fresh water. "Look at the damage we're doing to highly visible and beautiful resources like redwoods, whales, and coral reefs," Broad says. For all its importance, he explains, the invisibility of the underground world leaves it off the conservation priority list. So the expedition also made it a goal to publicize the importance of blue holes and the threats they face.

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