I’ve heard about “hydrothermal vents” deep under the sea that supposedly provide hydrogen, and bacteria can use that hydrogen as an energy source to feed on. But i’ve always been too lazy to read up on it. Not anymore!

https://en.wikipedia.org/wiki/Hydrogen-oxidizing_bacteria

This article mentions:

Occurring at ambient temperature, serpentinization is an exothermic geochemical mechanism that takes place when ultramafic rocks from deep in the Earth rise and encounter water. This process can produce large quantities of H2, as well as methane and organic substances.

This means, you have to imagine that there’s liquid (non-oxidated!!) metal inside the Earth’s core. Due to volcanic processes (and plate tectonics), it sometimes rises and comes into contact with the water from the ocean. Now, you have:

2 Fe + 3 H2O --> Fe2O3 + 3 H2

In this process 3 molecules of hydrogen were created. Or that’s the gist of it, if i understood correctly.

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The key insight is that raw, non-oxidated metal from the Earth’s interior is the reduction agent, and water is the substrate! Water gets split into oxygen and hydrogen, i.e. hydrogen gets reduced by the metal, and then gets released as a gas into the ocean -> atmosphere.