Extremophiles: caves rank among earth's most amazing natural wonders. And they brim with some of the planet's strangest life-forms

EXTREMOPHILES refers to microbes that thrive in extreme places. But the word might just as well apply to cavers who go anywhere to find such life-forms, like this ice cave in Greenland.

THE ROPE IS THEIR ONLY LIFELINE. CAVE EXPLORERS NANCY Holler Aulenbach and Hazel Barton Rappel (descend by rope) into an ice shaft, plunging 152.4 meters (500 feet) into frigid waters. They've rappelled into hundreds of caves--but never in Greenland and never an ice cave.

With ice picks in hand and sharp metal spikes called crampons strapped to their insulated boots, the cavers' descent is painstaking. One ill-placed spike can shatter the ice. "The sound is like a shotgun exploding in your face," says Aulenbach. What are they doing here? They're on scientific mission to map glacial caves and probe the ice for extreme forms of life--while their adventures are filmed for the IMAX movie Journey Into Amazing Caves.

Until recently, places like glaciers (moving ice fields) and pitch-dark caves seemed too inhospitable for life. Now, thanks to fearless explorers and powerful electron microscopes, scientists know that most caves teem with invisible life. These extremophiles are microscopic organisms that thrive in places too cold, too salty, too dark, too hot--too extreme--to support human or plant life. Scientists suspect extremophiles may even play a role in speleogenesis, or how caves form. And new research suggests these bugs might one day help cure diseases, break down toxic waste, even reveal the origins of life on Earth.

LIFE BELOW ZERO

SOME OF EARTH'S MOST ISOLATED caves lurk in the vast, moving ice fields that sprawl over Greenland. These glaciers reach a thickness of 3.2 kilometers (2 miles) and are thousands of years old. Most of Greenland is north of the Arctic Circle, yet during the brief summer, temperatures rise above freezing. Meltwater rivers, or freshets, rage over the ice and plunge into deep cracks, or crevasses, carving caves in the ice cap. "Here, ice caves form then disappear quickly,' says Aulenbach. "It's like geology at warp speed."

Since ice caves evolve quickly, a different set of caving rules applies than in rock caves. "In a rock cave, you wouldn't touch the formations," Aulenbach explains. But in an ice cave, it's vital to knock off icicles. "You don't want to be stabbed to death by a falling one."

As the explorers ease down the rope, blueness engulfs them. The whole cave glows blue because ice reflects the blue end of the light spectrum in sunlight. "It's like being inside a blue cathedral," Barton says. The deeper the cavers go, the older the ice. Alternating layers of white and blue ice make up the glacier: White layers form from winter snow, blue from surface ice that melted in summer and refroze.

Gummy Microbe

About 18.3 m (60 ft) down, Barton spies algae (plantlike microorganisms that produce their own food) in the 200-year-old ice. With sterile tools, she chops out a sample and seals the ice in a sample tube so she can study it under the microscope later. Earlier, on the surface, she sampled a gooey puddle and found a tardigrade, a microscopic animal resembling a gummy bear with a tubular mouth for sucking algae (see photo, bottom right). Tardigrades are extremophiles that can boast life spans of 200 years! "They've evolved to survive repeated freezing," Barton says.

Tardigrades produce a kind of "antifreeze" that prevents cell proteins (substances essential for growth) and membranes from breaking down even after years at subzero temperatures. The question is--how? Barton might begin to answer that when she returns to the University of Colorado's Pace Lab. Her scientific mission: "Looking at extremophiles for new disease-fighting medicines."

Barton has already found hundreds of new species so weird they're not even named--or identified on the Pace lab's massive "tree of life" chart. "Many extremophiles have natural lethal weapons that they use against each other in the fight for food," Barton says. "Our hope is to isolate an organism with such a weapon, to use against like tuberculosis or cancer." In fact, medical researchers have discovered extremophiles in New Mexican caves that spew chemicals to keep other extremophile species out of the way. Those chemicals attack and kill any foreign cell--including cancerous leukemia cells.

MEXICO'S HIDDEN WONDER

FROM THE ARCTIC circle to tropical jungles, Barton and Aulenbach trek the globe in search of amazing caves. "What drives me is the unknown," Aulenbach says. "What's around the next shadow?"

In that spirit, Barton squirms into scuba gear and plunges into a hidden pool in a remote Mexican jungle. At the bottom of the pool, or cenote, a tunnel snakes into a pitch-dark network of connected caves called Dos Ojos, the world's third-largest underwater cave system. The submerged caverns have lured hundreds of inexperienced cavers to their deaths.