WASHINGTON – While astronomers scour the skies for signs of life in outer space, biologists are exploring an enormous living world buried below the surface of the Earth.

Scientists estimate that nearly half the living material on our planet is hidden in or beneath the ocean or in rocks, soil, tree roots, mines, oil wells, lakes and aquifers on the continents.

They call it the “subsurface biosphere,” a dark world where the sun and stars don’t shine. Some call it Earth’s basement.

“Earth’s habitable zone extends to depths of hundreds or thousands of meters,” Katrina Edwards, a microbiologist at the University of Southern California, told a December conference of the American Geophysical Union in San Francisco. “The organisms that live in this environment may collectively have a mass equivalent to that of all of Earth’s surface dwellers and may provide keys to solving major environmental, agricultural and industrial problems.”

For example, geologists are considering whether to store some of the world’s excess carbon dioxide, a major greenhouse gas, in a worldwide network of crevices below the seafloor.

Scientists say research on “intraterrestrial life” complements astronomers’ hunt for “extraterrestrial life” around other stars and planets. The search for E.T. starts at home.

“Much that we do in our work to discover and understand the deep biosphere has relevance to the origin and search for life elsewhere in the universe,” Edwards said by e-mail. “Fundamentally, this is all about life detection. … Our inner space is a natural testing ground for outer space.”

To advance their understanding of subsurface life, marine geologists are about to launch three drill ship expeditions to punch holes in the seafloor and implant long-term scientific “observatories” linked by cable and satellite to onshore laboratories.

“We’ll be sitting in front of a fire hose of data,” said Andrew Fisher, a geophysicist at the University of California in Santa Cruz.

Subsurface biosphere research may shed light on the origin of life on Earth and the possibility of life on other planets.

“The conditions we see in the sub-seafloor are similar to what conditions may have been on the early Earth,” Fisher said. Similar conditions may exist or have existed on Mars or the moons of Jupiter.

“It is highly likely that if Mars supports life, it will also be in a deep biosphere where temperatures are high enough to allow liquid water,” John Parnell, a geologist at the University of Aberdeen, Scotland, told a conference of planetary scientists last week in The Woodlands, Texas.