There has been resurgent interest among U.S. researchers to find alien life forms, scientists say.

The Washington Post reported Sunday that there has been an explosion of interest in astrobiology, the search for extreme forms of life on Earth and for possibly similar life on other planets.

Biogeologist Lisa Pratt of Indiana University is among scientists who study so-called "extremophiles," or microbes found in very harsh Earth environments.

Pratt and her colleagues explored the depths of South African gold mines, where they found bacteria sustained only by the radioactive decay of nearby rocks.

"Until several years ago, absolutely nobody thought this kind of life was possible -- it hadn't even made it into science fiction," she said. "Now it's quite possible to imagine a microbe like that living deep beneath the surface of Mars."

Edward Weiler, one of the founders of NASA's astrobiology program, said there are likely other life forms out there.

"We now know the number of stars in the universe is something like 1 followed by 23 zeros," he said. "Given that number, how arrogant to think ours is the only sun with a planet that supports life, and that it's in the only solar system with intelligent life."