National Geographic : 2010 Aug
she analyzes water samples for gas levels with a portable spectrophotometer at the dive site. By comparing sul de densities with water depth, she's learning where di erent species of bacteria are likely to concentrate in a given blue hole and which mechanisms they use to survive. She is aided by Nikita Shiel-Rolle, a Bahamian cave diver and marine science graduate student at the University of Miami. Stargate's entrance lies on land that's been in her family for generations. "To give an idea of just how unique each hole is," Macalady says, "we analyzed the DNA of mi- crobes from ve inland blue holes and didn't nd any shared species." She is continually surprised by the variety of ways cave organisms harvest energy. "Some of these organisms use tricks we used to think were chemically impossible," she says. "If we can understand precisely how these microbes are making a living, we know what to look for on oxygen-free worlds." emerge from the hydrogen sul de into the black water below, my nausea and headache quickly pass. I'm relieved not to have to put into practice the suggested method for vomiting underwater, to say nothing of the impact my breakfast---a biological mushroom cloud---would have on the fragile environment. We descend slowly along the cave's east wall un- til a triangular portal appears in our lights: the entrance to a 2,500-foot-long tunnel known as South Passage. Stargate consists of a central sha some 340 feet deep, with passages extending north and south. Kakuk has explored North Passage about 1,300 feet out from the central sha , edging ever closer to the next blue hole to the north, and he's pushed even farther into South Passage. Of the more than one thousand blue holes believed to be in the Bahamas, less than 20 percent have been probed, and Kakuk estimates that three-quarters of those o er passages never seen before. e great age of Bahamian blue hole exploration lies ahead. e entry to South Passage is decorated by spectacular calcite formations, or speleothems, from drapery (thin, curtain-like formations) and straws ( ne, cylindrical deposits like soda straws) to the more familiar stalactites and stalagmites. (Remember, stalactites need to cling tightly to the ceiling above.) ey built up during ice ages, when the sea level dropped dramatically, leav- ing the caves dry. For Peter Swart, professor of marine geology and geophysics at the University of Miami, speleothems hold a priceless record of climate change in every year of their growth---at the inexorable rate of one to ve centimeters ev- ery thousand years. By studying speleothems in detail, Swart, Broad, and Amy Clement, a climate modeler at the University of Miami, will gain valuable information about sudden climate shi s of the past. ese include prolonged storms that blew Saharan dust across the Atlantic from Af- rica thousands of years ago, leaving high concen- trations of iron in the stalagmites and red stripes visible in the sediment of cave walls. Information In lightless blue holes, animals like this inch-long Agostocaris cave shrimp don't need surface pigmentation. Only part of the shrimp's digestive system has color. As living laboratories, blue holes are the scientific equivalent of Tut's tomb. From a diver's perspective, they're on par with Everest or K2.