National Geographic : 2010 Aug
• We sink into Stargate, sweeping the void with our dive lights. Fifty feet from the surface looms a pale haze, less smoky than fibrous, like a silvery net of faint, swirling cobwebs hovering motionless in the darkness. It's a layer of hydrogen sulfide, a toxic gas created by BY ANDREW TODHUNTER PHOTOGRAPHS BY WES C. SKILES In Stargate, a blue hole on Andros Island, divers illuminate North Passage (right). In Sawmill Sink (above), expedi- tion leader and anthropologist Kenny Broad descends through the bacterial layer on an exploratory dive. bacterial colonies and decaying organic matter. Divers entering the gas may experience itching skin, tingling, or dizziness; some smell rotten eggs as it penetrates their skin and metabolizes through their lungs. e gas density in Stargate is relatively low, but I'm struck by a wave of nausea as we de- scend. I glance at my guide, Brian Kakuk---one of the world's foremost cave divers. He appears unfazed. My head begins to throb; clearly, I'm unusually sensitive to the toxin. In the epic poem Beowulf, "dim serpent shapes" in the depths guard the lake of Grendel and his mother, shielding their lair. e otherworldly mist in Stargate appears to serve a similar role---a poisonous curtain that protects the deeper reaches of the cave. O shore ooded 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 envi- ronment on Earth, thanks largely to their geol- ogy and water chemistry. In these ooded caves, such as Stargate on Andros Island, the reduced tidal ow results in a sharp strati cation of water chemistry. A thin lens of fresh water---supplied by rainfall---lies atop a denser layer of salt water. e 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 sul de as a by-product. Known on land as swamp or sewer gas, hydrogen sul de in higher doses can cause delirium and death. As living laboratories, inland blue holes are the scienti c 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 some- thing 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.