Thousands of meters below the surface of the Pacific Ocean, researchers have come across something they didn’t expect. In an area called the Clarion-Clipperton Zone—known more for its mineral deposits than any signs of life—a team studying the seafloor recorded an increase in oxygen. That wouldn’t usually raise eyebrows, except for one thing: there was no sunlight, no plants, and no microbes producing it.
The discovery was made as part of the SMARTEX project, led by scientists from institutions including the National Oceanography Centre (UK), Heriot-Watt University, and the Scottish Association for Marine Science. Their goal wasn’t initially to search for unknown chemical reactions. They were studying the environmental impact of deep-sea mining, particularly in areas rich in polymetallic nodules—metal lumps on the seafloor containing manganese, cobalt, nickel, and other elements used in batteries and electronics.
While testing the sediment environment using sealed chambers on the seabed, the team noticed a small but consistent rise in oxygen levels over several days. At first, it didn’t make much sense. The chambers were placed in complete darkness, far below where sunlight can reach. And the presence of living organisms capable of producing oxygen through normal biological means—like photosynthesis or microbial activity—was either very limited or ruled out.
That’s when they looked more closely at the nodules themselves. These round metallic stones, scattered across the seabed, have an unusual property: their surfaces can support electrochemical reactions. In the right conditions, they seem to trigger a reaction that splits water molecules into hydrogen and oxygen—something similar to what happens in an artificial electrolysis setup.
This process doesn’t rely on any living thing or sunlight. Instead, it may happen because of the minerals in the nodules interacting with the surrounding seawater in the presence of small electrical gradients. The result is a release of oxygen in an environment where, by all traditional expectations, oxygen should not be forming on its own.
Scientists are calling this phenomenon “dark oxygen” because it occurs entirely in the absence of light. This finding raises new questions about how oxygen might have been created on early Earth, long before plants existed. It also opens the door to thinking differently about life on other planets or moons—especially those without sunlight but with water and rock.
Still, researchers are careful not to get too far ahead of the data. The experiments were carried out under specific conditions, and more studies are needed to confirm whether this process is widespread or only happens in certain places. It’s also not clear how much oxygen is being produced over time, or whether it has a large effect on the surrounding ecosystem.
What’s especially pressing is that these polymetallic nodules are already at the center of growing interest in deep-sea mining. Companies and governments want to extract the metals inside them to support green energy technologies, such as electric vehicle batteries. But disturbing the seafloor—particularly in areas that may have chemical processes we don’t yet understand—could have consequences we’re not prepared for.
The researchers behind this discovery are calling for more study and caution. They argue that the ocean floor holds more than just materials for mining—it may also play a quiet but important role in Earth’s chemistry. And if something as unexpected as oxygen can be produced in a place thought to be nearly lifeless, then we may need to rethink not just our approach to deep-sea mining, but our broader assumptions about how Earth works.
As this area continues to be explored, both scientifically and commercially, it’s likely we’ll learn more about how these deep-sea systems operate—and what we might be putting at risk by rushing to extract their resources. For now, “dark oxygen” is a clue to a part of Earth’s chemistry that stayed hidden for millions of years. What else might be down there is still an open question.
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