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As Kakuk and I emerge from the hydrogen sulfide 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 until 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 shaft some 340 feet deep, with passages extending north and south. Kakuk has explored North Passage about 1,300 feet out from the central shaft, 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 offer passages never seen before. The great age of Bahamian blue hole exploration lies ahead.

The entry to South Passage is decorated by spectacular calcite formations, or speleothems, from drapery (thin, curtain-like formations) and straws (fine, cylindrical deposits like soda straws) to the more familiar stalactites and stalagmites. (Remember, stalactites need to cling tightly to the ceiling above.) They built up during ice ages, when the sea level dropped dramatically, leaving 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 five centimeters every 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 shifts of the past. These include prolonged storms that blew Saharan dust across the Atlantic from Africa thousands of years ago, leaving high concentrations of iron in the stalagmites and red stripes visible in the sediment of cave walls. Information from speleothems will shed light on today's rapid warming and the associated rise of sea level. "The better we understand how the natural climate system works," Swart says, "the better we can understand the nature and degree of our own impact."

At Kakuk's direction, I tie off our safety reel to the line at the entrance of South Passage and follow him inside. In the play of our lights, the natural geometry of the corridor is breathtaking. Above soars a vaulted, triangular ceiling; below, a floor of impenetrable darkness. There is an eerie quality of intention—the vaulted corridor seems more designed than randomly occurring—and I'm reminded simultaneously of the outer walls of Mycenae and the gallery in Khufu's Great Pyramid. Covering my light with my palm, I hover and watch Kakuk's single lamp move steadily forward as the walls' steep angles come into view. I had expected a measure of anxiety in such an alien environment, but for all its unearthly surrealism, this motionless, lightless place is profoundly calming. For a moment I relax completely, releasing an attenuated breath and swinging my light upward through the swarm of ascending bubbles.

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