D Waltham, Could Europe be the Saudi Arabia of natural hydrogen, New Energy World, February 2025, p 5

The pursuit of sources of natural hydrogen stretch across over 20 countries, and in some cases the potential sources have been indicated by ‘fairy circles’ of disturbed vegetation. The requirements for such deposits are source rock, reservoir rock, cap rock, migration route and good timing. Mantle rocks generate hydrogen when they are warm and wet with copious free water. One example concerns the Pyrenees mountain layer, where natural hydrogen has been found on both the French and Spanish sides. Natural hydrogen exploration is a classic high-risk situation, but the economics of success are enormous.