Many chemical reactions produce hydrogen in nature, but The most interesting process, because it allows the generation of very large quantities of gas, is the alteration reaction of water on rocks from the coat. These are in fact composed mainly of a mineral called olivine which, upon contact with water, will transform into serpentine.
Now, this alteration reaction called serpentinization will also produce hydrogen. One might think that this process is not of great interest since we are talking about rocks from the mantle and not from the crust. And so far, it has not been possible to drill down into the mantle.
Except that there are certain areas where mantle rocks can be exhumed to the surface. They are found in certain very specific tectonic contexts, notably in certain mountain ranges resulting from the collision of continental plates.
The researchers identified in which cases the serpentinization conditions were met to be able to lead to the formation of hydrogen reservoirs. In order to take place optimally, serpentinization requires temperatures between 200 and 350 °C and, of course, the presence of water. Conditions achieved in mountain ranges where the numerous networks of fractures and flaws allow water to circulate to the depths where these temperatures are reached. In addition, these environments have reservoir rocks such as Stoneware in which hydrogen can accumulate and be stored over time.
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https://www.futura-sciences.com/planete/actualites/montagne-nos-montagnes-pourraient-etre-enormes-reservoirs-hydrogene-naturel-119751/