Worms have been found living at depths in the Earth where it was previously thought animals could not survive.
Discovered in South African mines, the roundworms can survive in the 48°C water that seeps between cracks 1.3 kilometres beneath the Earth's crust, the BBC reports.
The find has surprised scientists who, until now, believed only single-celled bacteria thrived at these depths.
Writing in the journal Nature, the team says this is the deepest-living multi-cellular organism known to science.
The researchers found two species of worm - one is a new species to science, which the scientists have named Halicephalobus mephisto after Faust's Lord of the Underworld.
The other is the previously known roundworm Plectus aquatilis.
The water in which the worms were found is between 3000 and 10,000 years old, and so it is unlikely that the researchers brought the worms with them into the mines.
The scientists believe that the animals may have originally been on the surface but got washed down into the cracks in the Earth's crust by ancient rainwater.