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We instinctively associate life with oxygen, but living things existed on Earth for more than a billion years in the absence of the one gas divers can't last minutes without. Ironically, the oxy­gen revolution came about through the rise of bacteria that created oxygen as a waste product. Jenn Macalady, an astrobiologist at Pennsylvania State University's Department of Geosciences, is studying the water chemistry of Bahamian blue holes to understand the conditions most similar to the earliest, oxygen-free environments that supported life. She's especially interested in the period from about four billion years ago—when life first appeared on Earth—to what scientists call the oxygen revolution, some 2.5 billion years ago. By investigating bacteria that thrive in the anoxic waters of blue holes, she can postulate what may exist in the oxygen-free, liquid-water environments of distant planets and moons. "The universe is made of the same elements," Macalady says, "and habitable planets are likely to share many of the same characteristics, like a temperature range conducive to life and the presence of water." Many astrobiologists believe such conditions may exist in pockets of liquid water deep beneath the surface of Mars or in a sea under the frozen crust of Jupiter's moon Europa—to say nothing of far distant worlds potentially much more like our own.

Macalady doesn't dive, but she's an active dry caver who hauls tanks, coils ropes, and chats with young Bahamians about cave slime and the possibility of life in the universe. At her direction, divers take water, bacteria, and hydrogen sulfide samples at depths ranging from the surface to 270 feet. Most of her studies—including DNA testing, bacterial culturing, and the search for molecular fossils—must wait until she gets back to the equipment in her lab. But hydrogen sulfide is too unstable to transport, so she analyzes water samples for gas levels with a portable spectrophotometer at the dive site. By comparing sulfide densities with water depth, she's learning where different 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 microbes from five inland blue holes and didn't find 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."

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