Miles beneath the Earth's surface, where no light or air reaches, tiny organisms are eking out a meager existence.
Yet despite making up an estimated 6 percent of all life on Earth, researchers know almost nothing about these deep-dwellers. And scientists have failed to culture, or grow, the bacteria in the lab, making it difficult to understand how they survive the harsh, energy-starved environment below the planet's surface.
"We're asking really basic, fundamental, big-time questions: Who is there? What are they doing? How did they get there? How many of them are there?" said Jan Amend, an earth scientist at the University of Southern California's Center for Dark Energy Biosphere Investigations. "These are really, really simple questions but very fundamental ones we don't know the answers to."
To answer some of these questions, scientists have embarked on a census to catalog the life buried beneath the Earth's surface. What they find could help them understand the origins of life on Earth, or reveal the kinds of life that could survive on other planets. (7 Theories on the Origin of Life)
Over the last several decades, researchers have probed the microbial communities living on the seafloor, then gradually pushed beneath the surface. Deeper and deeper, scientists still found life. The deepest life yet found are bacteria living 2 miles (3.2 kilometers) below the surface in South African gold mines. (And in 2011, scientists even found worms that live underground and eat those bacteria.)
But bacteria and archaea have been found in sediments in hydrothermal vents, subglacial lakes, mud volcanoes, underwater mountains and many other environments, said Rick Colwell, a microbiologist at Oregon State University, who presented results from a new census of such organisms earlier this month at the American Geophysical Union meeting in San Francisco. Everywhere researchers look, the subsurface is teeming with life.
To begin to catalog these communities, Sharon Grim of the Marine Biological Laboratory in Woods Hole, Mass., and colleagues such as Colwell, with the Census of Deep Life, have begun analyzing genetic data from all the underground archaea and bacteria they can, including a key identifying set of genes.
"It's like an organism's dog tags, it indicates to a rough extent who they are," Colwell told LiveScience.
Though the results are still early, they are finding that the life at that depth is incredibly diverse, Colwell said.