He and his colleagues carried out their research in an area of the deep sea between Hawaii and Mexico - part of a vast swathe of seafloor that is covered with these metal nodules. The nodules form when dissolved metals in seawater collect on fragments of shell - or other debris. It's a process that takes millions of years.

And because these nodules contain metals like lithium, cobalt and copper - all of which are needed to make batteries - many mining companies are developing technology to collect them and bring them to the surface.

But Prof Sweetman says the dark oxygen they make could also support life on the seafloor. And his discovery, published in the journal Nature Geoscience, raises new concerns about the risks of proposed deep-sea mining ventures.