Creatures dubbed "worms from hell" have been found at a depth of more than a mile, where it was thought animals could not survive, U.S. researchers say.

The nematodes, or roundworms, were found in gold mines in South Africa, The Washington Post reported Thursday.

Researcher Tullis Onstott of Princeton University, working with Gaetan Borgonie of the University of Ghent in Belgium, said the discovery of creatures that far below ground with complete nervous, digestive and reproductive systems, was akin to finding "Moby Dick in Lake Ontario."

"This is telling us something brand new," said Onstott, a pioneer in the study of microbial life known generally as extremophiles living in environments long believed to be uninhabitable.

"For a relatively complex creature like a nematode to penetrate that deep is simply remarkable," he said.

The nematode findings could have important implications for astrobiology, the search for extraterrestrial life, Onstott and Borgonie said.

On Mars, in particular, if life did originate, and if it had sufficient time to go underground deep enough to survive worsening conditions, "then evolution of Martian life might have continued underground," Borgonie said.

"Life on Mars could be more complex than we imagined," Borgonie said.