“I first saw this in 2013,” says the study’s lead researcher, Professor Andrew Sweetman, from the Scottish Association for Marine Science (SAMS). “A huge amount of oxygen was being produced on the seafloor in complete darkness.”

I did not take this into account, because I knew that oxygen is obtained by photosynthesis.

“Over time, I realised that I had ignored this huge discovery for years,” Sweetman told BBC News.

He and his colleagues conducted their research in an area of ​​deep water between Hawaii and Mexico, part of a large area of ​​seafloor covered in mineral nodules.

Nodules form when minerals dissolved in seawater accumulate on shell fragments or other debris. This process takes millions of years.

Because these nodules contain minerals such as lithium, cobalt and copper (all of which are essential for making batteries), many mining companies are developing technologies to collect them and bring them to the surface.

According to Professor Sweetman, the black oxygen produced by the nodules could also support life on the seafloor. His discovery, published in the journal Nature Geoscience, raises new concerns about the risks associated with deep-sea mining projects.

Scientists have discovered that mineral nodules can produce oxygen precisely because they act like batteries.

“If you put a battery in seawater, it starts to form bubbles,” says Professor Sweetman.

“This is because the electric current separates the seawater into oxygen and hydrogen (which are the bubbles). We think this is what happens to these nodules in their natural state.

“It’s like a battery in a torch,” he added. “You put one battery in and it doesn’t light up. If you put two together, you have enough voltage to power it. When the nodules are in contact with each other on the seafloor, they work in unison, like multiple batteries.”

The researchers tested the theory in the lab, collecting and studying potato-sized metal nodules. They measured the voltage, or electrical current, across the surface of each metal block. They found that the voltage was roughly the same as a typical AA battery.

This means, they say, that seafloor nodules can generate electric currents large enough to split or electrolyze seawater molecules.

Researchers believe that the same process — producing oxygen through electricity without light or biological intervention — could occur on other planets and moons, creating oxygen-rich environments where life could thrive.

The Clarion-Clipperton Zone, where the discovery was made, is already being explored by several seabed mining companies that are developing technology to collect the nodules and bring them to a surface vessel.

The US National Oceanographic and Atmospheric Administration (NOAA) has warned that deep sea mining could “destroy life and habitats on the seafloor.”

Similarly, more than 800 scientists from 44 countries have signed a petition highlighting the environmental risks of the practice and calling for a moratorium.

It is common to discover new species in the depths of the ocean. It is often said that we know more about the moon than we do about the ocean floor.

This new discovery suggests that the nodules themselves could provide the oxygen needed to sustain life in this part of the planet.

Professor Murray Roberts, a marine biologist at the University of Edinburgh, was one of the scientists who signed the petition.

“There is already overwhelming evidence that deep-sea nodule mining will destroy ecosystems that we barely understand,” he told the BBC.

“Given that these fields cover vast areas of our planet, it would be foolish to pursue deep-sea mining knowing that they could be an important source of oxygen production.

“I don’t think this study will end mining,” Professor Sweetman added.

“But we need to study it in more detail and use the information and data we collect in the future if we are to go deep into the ocean and exploit mining resources in the most environmentally friendly way possible.