They are among the toughest creatures known to science, and species like them could make up two thirds of all the living material on the planet.

Researchers have discovered living microbes in rock 1.6 km under the sea bed, which itself has 4.5 km of seawater above it. The bacteria-like creatures, which were found in sediment cores extracted by a ship off the coast of Newfoundland in Canada, are able to survive extreme pressure and temperatures of up to 100C. The previous record for life was 842 m underground.

The creatures are single-celled organisms called archaea which are able to thrive in an ecosystem where sunlight never penetrates. The researchers who discovered them are unsure exactly how they do this, but one possibility is that they effectively eat oil.

The heat cooks hydrocarbons in the sediment, said Prof John Parkes at Cardiff University in the UK, who led the study, breaking them down into methane and other chemicals. "It's an intuitive guess that they might well be using these compounds, but we can't be sure," he added.

How they got there is also a mystery. One possibility is that a population of bacteria has remained in the rocks ever since they were formed at the surface 111m million years ago. Another is that they migrated there in underground water currents.

Bringing the creatures to the surface will have killed them, but the scientists believe they thrive in their own extreme environment.

"All the evidence suggests they are viable, active organisms," said Parkes, who reported the discovery in this week's issue of the journal Science. The team analysed DNA from the microbes and found that it had not been broken down into small pieces, as would be expected at high temperature. They were also able to see the cells in the process of dividing when they peered at them down a microscope.

DNA analysis suggests that the microbes' closest relatives live in deep-sea hydrothermal vents.

The discovery of life-forms so deep underground strengthens the notion that most of the organisms on the planet are in fact hardy, single-celled creatures that live in the rock beneath our feet. Researchers estimate that two thirds of all the biomass on the planet is underground.