A dark realm far beneath the Earth's surface is a surprisingly rich home for tiny worms and "zombie microbes" that may hold clues to the origins of life, scientists said on Monday.

"It's an amazing new world," said Robert Hazen, head of the Deep Carbon Observatory, a decade-long $US500 million project to study the planet's carbon, an element vital to life and found in everything from oil to diamonds.

"It's very possible that there's a deep microbial biosphere that goes down more than 10 km (6 miles), maybe 20," Hazen told Reuters of the first book by the Observatory, published on Monday and written by more than 50 experts in nine countries.

Microbes had been reported, for instance, in rocks recovered by drilling more than 6 km below the surface in China's Songliao basin, he said. And tiny worms have been found in fractures in rocks 1.3 km deep in a South African mine.

The single-celled microbes found deep underground include bacteria, which need water and nutrients to grow but not necessarily oxygen, and archaea, which can live off compounds such as ammonia or sulphur.