Cenotes del Rancho La azufrosa

"Thirteen years ago on a sunny spring morning, two divers prepared to descend into what could be the world's deepest water-filled pit: northeastern Mexico's El Zacatón, a 180-meterwide limestone sinkhole filled by hydrothermal springs. The water is 30°C, teeming with strange microbes, and pitch-black below the first 30 meters. One diver was Sheck Exley, then holder of the world's scuba depth record; the other was his friend Jim Bowden, a top underwater caver. They wished each other luck, adjusted their masks, and began free-falling down separate safety lines. Ten hours later, Bowden surfaced with a new world record--925 feet (282 meters)--without ever finding the bottom. Exley did not surface. Three days later, his body was pulled out, tangled in the line. No one knows what killed him. The system owes its vastness to volcanism that adds heat and gases to water running into the limestone. This hastens chemical dissolution of the rock as well as making things cozy for unusual bacteria.The greatest challenge at Zacatón may be finding and sampling organisms. It's no problem on top: Along with little fish, water moccasins and other snakes up to 2.7 meters long cut the surface faster than humans can swim. But below about half a meter, the hot, chemical-laced water lacks both oxygen and conventional aquatic life. Divers have found a shallow tunnel connecting Zacatón to a nearby river that holds the bones of countless turtles; like Exley, they may have dived too far or too long. On the other hand, hydrogen sulfide and other volcanic gases feed thriving communities of extremophile microbes. Each morning the water is clear, but by noon it turns milky gray, probably from elemental sulfur precipitated out by photosynthetic sulfide-eating bacteria. Further down, the walls are lined with spongy red and purple microbe mats, says the team's microbiologist, John Spear of the Colorado School of Mines in Golden. In the first 82 meters--as far as human divers dare sample--Spear has spotted 27 divisions of bacteria, including six that may be new, along with archaea and planktonic diatoms. "The diversity is astounding. I think that if we get down further, there will be even more," he says. He expects only microbes but does not rule out bigger life forms. "We could run into tubeworms, or crabs, or something else. We really have no idea.""