By Alister Doyle

OSLO (Reuters) - Scientists will start drilling off Japan this month to seek the hottest place where life can survive in a hellish uncharted realm deep below the seabed.

The drilling under the Nankai Trough in the Pacific Ocean will be part of a project by 900 experts to map carbon underground, hoping for clues to everything from the origin of life on Earth to the formation of oil and gas.

Previously, microbes have been found living at a torrid 121 degrees Celsius (249.8°F) around a volcanic vent on the seabed in the Pacific Ocean off the United States.

Scientists will now drill into rocks where temperatures reach 130°C (266°F) in a two-month trip off southern Japan starting on Sept. 12, said Kai-Uwe Hinrichs, of the University of Bremen in Germany who led the scientific proposal for the mission.

He reckoned life was likely to exist at temperatures around a maximum 85°C to 90°C (176°F-194°F) beneath the surface. He said there was probably less food in such rocks, heated by the molten core of the Earth, than near volcanoes on the seabed.

"But we've been surprised in these systems before. I wouldn't bet any money on it," he told Reuters.

Water in the Nankai Trough is 4.7 kilometers (2.92 miles) deep and the scientists will drill another 1.2 kms into the Earth. The researchers reckon it is easier to prevent contamination of samples on a drilling ship than on land.

Scientists say that they are discovering vast amounts of carbon-based life in the little understood subterranean zone.

Still, they reckon deep rocks are too disconnected from the surface to be exploited by humanity to soak up carbon dioxide, the main greenhouse gas emitted by human activities.

And the temperature limits of life are unclear.

"My guess is that 121°C is not the highest temperature at which life can grow and replicate," Derek Lovley, a U.S. scientist who helped identify Strain 121, the microbe that can reproduce at 121°C.

Lovley, of the University of Massachusetts in Amherst who is not involved in the Japanese project, told Reuters that it might be possible to adapt Strain 121 to higher temperatures and that other unknown life forms may live in even hotter places.

Guidelines for sterilizing surgical instruments in hospitals often stipulate 121°C. Heat-loving microbes like Strain 121, however, would die of cold if exposed to a human body at 37°C.

(Link to mission: https://deepcarbon.net/feature/how-hot-is-too-hot#.V9ExO_l97mE; Reporting By Alister Doyle; Editing by Toby Chopra)