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A mineral formed by biological processes has been found in rocks that were 12 miles (20km) beneath Earth's surface 100 million years ago.

If the finding is confirmed, it will be among the deepest signs of life ever discovered.

And it could also have implications for life on other worlds, suggesting that more regions of Earth are habitable than we anticipated.

Scientists from Yale University have found evidence for ancient underground life. A mineral called aragonite (stock image shown) was found in rocks that are now on the surface. A specific mix of carbon isotopes suggests the aragonite found came from subsurface methane made by microorganisms

The finding was made by a team from Yale University on Washington state's Lopez Island.

The mineral found in the rocks was something known as aragonite, which can form both biologically and by natural processes.

However, in this instance the scientists found a specific mix of carbon isotopes that indicate it has biological origins.

The findings suggest this aragonite came from methane made by microorganisms.

'These really light signals are only observed when you have biological processes,' Philippa Stoddard, an undergraduate at Yale University, told Live Science.

The aragonite was found in an outcrop 165ft (50 metres) long that lies in a fault zone 1.9 miles (3km) thick called the Lopez Structural Complex.

It's thought the life may have originated in a subduction zone deep beneath the surface of Earth where two tectonic plates came into contact.

Here, it may have been possible for water to be embedded in the subsurface rocks.

Over 100 million years the rocks were gradually pushed upwards and are now seen on the surface.

Exactly what sort of life it would have been is not known, but it would likely have been simply bacteria or other simple microorganisms.

If life did survive at these great depths, it would have had to endure temperatures of more than 122°C (252°F).

High pressure may have enabled life to live even at such high temperatures underground, but nonetheless it would be an extreme environment in which microorganisms could survive.

And discovering life in such environments could aid scientists in their hunt for life on other planets besides Earth.

The discovery was made in an outcrop of rocks on Washington state's Lopez Island (shown). 100 million years ago these were 12 miles (20km) underground before being gradually brought to the surface. Life could have survive in the rocks between tectonic plates

It's thought the life may have originated in a subduction zone (shown) deep beneath the surface of Earth where two tectonic plates came into contact. Here, it may have been possible for water to be embedded in the subsurface rocks. Over 100 million years the rocks were gradually pushed upwards

As life is found in more and more extreme environements, it raises hopes that some form of past or present microbial life may be found on other worlds in our solar system besides Mars.

And it may ultimately force us to rethink the characteristics that enable us to label a planet beyond the solar system 'habitable', suggesting even more may be habitable than thought. 

'At low surface pressures, bacterial life is know to remain active to temperatures of about 122°C [252°F],' the researchers write.

'Biomolecules are stabilised by pressure, so bacterial life should extend to higher temperatures within the Earth's interior.

'We suggest that the Lopez Island aragonite veins are evidence of this deep life.'

The team presented their findings at the Geological Society of America's annual meeting in Vancouver, British Columbia.

Life has already been found in a variety of bizarre environments on Earth. Here tiny crabs and other sea life live next to a hot hydrothermal vent on the ocean's floor. The heat and minerals expelled by the vent allow these creatures to survive without sunlight. The latest discovery could have implications for finding alien life