National Geographic : 1970 Sep
When the boatman saw our heads break water, he stared in disbelief, but his expres sion was almost comical when we drew up his dripping motor. But no question: It was his motor and there we were, alive and well. Watery Caves Were Once Dry That was more than ten years ago, and it was my first dive into a blue hole off Andros. Since that time I have made hundreds more. I am by profession a research chemist, but underwater exploration and photography are my major avocations; whenever I can get away from my laboratory in Toronto, I go to Andros to dive. Through aerial surveys over the past decade, I have charted several hun dred locations which might prove to be blue holes, and I have entered and explored 54 of them, some to depths of more than 200 feet. Ever since my first encounter with these strange holes, I have felt irresistibly drawn toward their dark mouths. Everyone talked about the blue holes, but no one, apparently, had mustered either the equipment or the curiosity to explore them. Anyone who dives into the blue holes soon begins to wonder how they were formed. Ge ologists, from studies of marine limestone deposits, can tell us something about them. The Bahamas are the above-water bits of a chain of limestone platforms, or banks, stretching in a 750-mile arc southeast of Florida (inset map, page 349).* Andros, where we have found the largest number of blue holes, is the largest island of the chain, a hundred miles long and a maximum of forty wide. Andros lies on the edge of the Great Bahama Bank with its back to the shallows, its eastern face lapped by the Tongue of the Ocean (pages 356-7), a dark abyss dropping to 6,000-foot depths. The Bahama platforms began to form at least 130,000,000 years ago. In the past mil lion years or so, the colder periods of the Ice Age trapped water in enormously enlarged glaciers and icecaps, thus lowering the level of the sea hundreds of feet and exposing the platforms. In warmer periods water was grad ually released as the ice melted, and the sea crept up again. It was during times of low water that the blue holes were formed-most probably as ordinary dry-land caves or sinkholes. Heavy rains-rain is slightly acid-ate away the basic limestone, opening fissures and form ing underground pockets. Flowing water fol lowed the meandering crevices and enlarged them, turning the pockets into caverns. As the last glacial period ended, once again rais ing the level of the sea, the caverns flooded, forming the blue holes of today. *See "The Bahamas: More of Sea Than of Land," by Carleton Mitchell, GEOGRAPHIC, February 1967. Tendrils of color float through the hole at Rat Cay as George Jr. releases fluorescein dye to study the currents. The author be lieves tidal pressure forces sea water beneath Andros's fresh-water table and raises it; at ebb tide, the table may act as a piston, flush ing the holes in a torrent flowing at speeds as high as three knots. Blizzard of fish, swirling pilchards screen a hole in Conch Bay. "As we passed," said the author, "they formed a tunnel that closed behind us." Three snappers eye the photog rapher. Groupers, jacks, lobsters, and crabs also seek sanctuary in the shadowy clefts. 352 KODACOLORS BY UGEREt J. BtNJAMIN l N.t. .