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"We have instruments that can go to the deepest parts of the ocean," explained Prof Sweetman.

"If there's oxygen," said Prof Sweetman, "there could be microbial life taking advantage of that."

“I first saw this in 2013 - an enormous amount of oxygen being produced at the seafloor in complete darkness,” explains lead researcher Prof Andrew Sweetman from the Scottish Association for Marine Science.

But Prof Sweetman says the dark oxygen they make could also support life on the seafloor.

“If you put a battery into seawater, it starts fizzing,” explained Prof Sweetman.