A diverse range of life forms exists deep below Earth's surface, scientists have concluded, but they survive at an incredibly slow pace.
Long-lived bacteria, reproducing only once every 10,000 years, have been found in rocks 2.5km (1.5 miles) below the ocean floor that are as much as 100 million years old.
Viruses and fungi have also been found.
The discoveries raise questions about how life persists in such extreme conditions.
Scientists from the Integrated Ocean Drilling Program have announced the findings at the Goldschmidt conference, a meeting of more than 4,000 geochemists, in Florence, Italy.
Fumio Inagaki of the Japan Agency for Marine-Earth Science and Technology, reported that the microbes exist in very low concentrations, of around 1,000 microbes in every tea spoon full of rock, compared with billions or trillions of bacteria that would typically be found in the same amount of soil at Earth's surface.