If it's just us in this universe, what a terrible waste of space. For thousands of years, humans have wondered about who and what might be living beyond the confines of our planet: gods, beneficent or angry; a heaven full of sinners long forgiven; creatures as large and strange as our imagination.

Some scientists now are on the cusp of bringing those musings back to Earth and recasting our humanity yet again. "Astrobiology" is the name of their young but fast-growing field, which immodestly seeks to identify life throughout the universe, partly by determining how it began on our planet. The men and women of astrobiology -- an iconoclastic lot, quite unlike the caricatures of geeks in white lab coats or UFO-crazed conspiracy theorists -- are driven by a confidence that extraterrestrial creatures are there to be found, if only we could learn how to find them. Most astrobiologists hold the conviction that if a form of independently evolved life, even the tiniest microbe, is detected below the surface of Mars or of one of Jupiter or Saturn's larger moons, the odds that life does exist elsewhere in our galaxy and, potentially, in billions of others, shoot up dramatically. A solar system that produces one genesis -- ours -- might be an anomaly. A single solar system that produces two or more geneses tells us that life can begin and evolve whenever and wherever conditions allow, and that extraterrestrial life may well be an intergalactic commonplace.

With goals so enormous and compelling, astrobiology has brought forth a new generation of outside-the-box researchers, field scientists, adventurers and thinkers -- part Carl Sagan, part Indiana Jones, part Watson and Crick, part "CSI: Mars." They are men and women who drop deep below the surface of Earth or tunnel into Antarctic glaciers in search of life in the most extreme places, who probe volcanoes for clues into how Earthly life began, who propel life-detecting robots into deep space and who will ultimately send colleagues to other planets. These explorers come up with ever more ingenious methods for detecting planets that circle distant suns; they scour our planet for Mars-like habitats they can minutely study for the life-supporting conditions astronauts might encounter when our spaceships arrive there. They probe the cosmos as far as 13 billion light-years away for signs of the earliest stirrings of the order and chemistry that created life on Earth. Some are even working to define and understand "life" by creating it in the lab.

These scientists have harnessed that childhood excitement so many of us felt when, on hot, hard-to-sleep summer nights, we tried to imagine what it would be like to visit Mars (very dry), or travel to the end of the universe (very confusing), be around when life first began (very lonely), or come across extraterrestrial life (most thrilling of all). The world has changed enough that today, a large and growing number of scientists are earning their livelihoods turning their imaginings into hypotheses and putting them to tests inconceivable even a decade ago.

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Science moves ahead on hunches. Tullis Onstott, a Princeton University geobiologist, first descended into a South African gold mine on a hunch in 1996, using $6,000 of his own money and carrying, instead of the usual pickaxes and dynamite, a small hammer, a chisel, some vials for collecting water, and some sterilized bags for collecting rocks. Over the next decade, he and his fellow mine divers found microbes that broke nearly every rule of life. Until then, it was taken as scientific fact that to survive, a creature needs an energy source and an environment that isn't extremely hot or cold; isn't overly acidic, alkaline or salty; isn't suffused with radiation; or isn't under great pressure. Creatures also need to reproduce or split with some regularity. On his first trip into the mines, On-stott found microbes living as far down as two miles that struck out on virtually all of these counts. His prized discovery, made a few years later and confirmed in 2006, was of a bacterium nourished by food -- molecules, actually -- split apart by energy released by the radioactive decay of surrounding rocks. The microbe also needs some minerals to survive and some water, which is hidden from human view until miners open up tunnels and bore holes, tapping into underground lakes, streams and even tiny fissures within the rocks. Not only do these microbes live and move around miles below the surface, but also they seem to split -- that is, reproduce -- as seldom as once a century.

A reading of the genome of Onstott's astounding bacterium, as well as analysis of the "age" of the water that is often its home, says that the microbe has not seen the light of day, or interacted with anything produced from sunlight, for perhaps up to 40 million years. But it has DNA, reproduces and is clearly alive. The researchers who sequenced its genome found that the microbe has highly unusual abilities to take in needed carbon and nitrogen from nonliving sources -- very useful abilities, given the absence of carbon-based life in its isolated and unrelentingly harsh environment. It even had genes for a tail of sorts, a whiplike growth that would allow it to swim to hidden sources of nourishment. The bug, Onstott concluded, is widespread in a 130-mile-long subterranean region of the gold belt of South Africa. To honor the creature and the world to which it long ago traveled and made its home, the team (co-directed by geologist Lisa Pratt of Indiana University) sought a name in line with the achievement -- first of the bug's existence, and then their discovery of it. They found it in the secret Latin inscription on a scrap of parchment that Professor von Hardwigg, hero of the Jules Verne classic "A Journey to the Center of the Earth," comes across at the beginning of the book. The parchment directs him to a volcano in Iceland and tells him: Descende, audax viator, et terrestre centrum attinges (Descend, bold traveler, and you will attain the center of the Earth). And so the world was introduced to Desulforudis audaxviator, extremophile par excellence.

South Africa is today the center of Onstott's research not because similar microbial life doesn't exist far below New York or London or Tokyo, but simply because it is where the deepest mines have been dug. Onstott had first explored the deep underground for microbes as part of a Department of Energy drilling program in Savannah, Ga., and later at a Texaco well site in western Virginia. Frustrated by his limited results and fearing contamination in his samples, he cast around for alternatives and landed on South Africa's gold, platinum and diamond mines -- with shafts descending two miles and more. But mine owners were reluctant to let strangers into their domains. It took Onstott and others two years of negotiating to get into the mines to later achieve their breakthroughs. Today, he and Esta van Heerden, the head of the Extreme Biochemistry research group at the University of the Free State in Bloemfontein, have won the confidence of the people who run many of the mines of the Witwatersrand Basin, the most productive in the world. When a potentially interesting section of mine is opened, or is going to be shut in forever, the mine operators now call van Heerden to give a heads-up.

Their cooperation has been a godsend to astrobiology and has led Onstott and others to conclude that D. audaxviator and untold trillions of other underground microbes also live miles below your shopping center, your bedroom, your favorite national park. Or miles below the surface of Mars, for that matter. Eons ago, our most similar planetary neighbor was far more hospitable to life than was Earth, which had endured the collision with a smaller planet that produced the moon. But Mars somehow lost its magnetic field, its atmosphere and, thus, its ability to hold liquid water on its surface or to protect against solar radiation and deadly ultraviolet light. Mars scientists have long speculated that primitive organisms met the new challenges by descending below the surface and adapting through a desperate evolution. Now, living proof exists of a potentially parallel scenario on Earth.

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I wanted to see this proof for myself, or, at least, see its subterranean home. That's how I found myself suiting up for a descent into a mine owned by Northam Platinum, a sprawling operation in the northern bush of South Africa, just beyond the aptly named Crocodile River. The daily routine on the surface was ordered, polite and matter-of-fact. Geologists in their white coveralls inspected new equipment; managers made sure the shifts were coming and going as planned. Even the miners, lined up in long rows waiting for the trip down to their work, were smiling and chatting; and some were swaying to the bouncy music coming from the loudspeakers. The grass was clipped; the grounds were clean; the 5,000-worker plant was humming.

Our group of scientists was outfitted in coveralls highlighted with fluorescent striping, heavy rubber boots and goggles, hard hats capped with a miner's light, and a mandatory safety kit strapped to our belts that included a breathing device that can filter out carbon monoxide. The chatter ended as we were ushered into the manager's cage; the miners piled into another. The doors slammed shut, and both cages picked up speed, plunging down to Level 7 -- a 30-mph express ride into Earth's crust, accompanied by the sound of falling rocks hitting our carrier as we sped by. We jolted to a stop, an attendant pried the door half open, and we stumbled out into a high-ceilinged, man-made chamber that housed a railyard for miniature ore trains. Accompanied by an array of mine officials, we passed through this small island of light and set off for the outer reaches of Level 7. We were 1.1 miles below ground and surrounded everywhere by dark, gloomy rock.