The challenges of the green energy transition to address the ever-increasing effects of climate change has raised the profile of naturally-occurring geologic hydrogen accumulations in the crust – but the foundational research on this phenomenon arose from investigations focused on life forms other than humans. Only recently have human populations begun to consider competing with their microbial cousins for this subsurface hydrogen resource on a global scale. Over the past few decades, Earth analogue studies has revealed previously unexplored localities and unexpected processes that have challenged us to think more broadly and universally about the fundamentals of habitability. Scientists investigating microbial communities identified water-rock chemical reactions such as serpentinization and radiolysis that produce critical electron donors (e.g. hydrogen) and electron acceptors (e.g. sulfate) capable of sustaining chemolithotrophic microbial communities in the oceanic and terrestrial crust. Such processes of water-rock reaction have now been shown to be a major driver H and S cycles in the subsurface of the planet, and increasingly their role in the deep C cycle is being investigated.

Subsurface “rock-eating” microbial communities have been shown to be sustained on long time scales, isolated from the surface hydrologic cycle. Both field and laboratory discoveries are expanding the spectrum of water-rock reactions that drive the H, S and C deep cycle and provide mechanisms for sustaining deep subsurface life in the absence of interaction with a surface photosphere. Discussions of habitability typically focus on the necessity for fluid mixing and/or spatial geochemical gradients, but recent discoveries suggests apparently thermally and spatially “stagnant” systems may still be habitable through radiolysis. Taken together, insights from terrestrial analogue sites suggest new potentialmodels for planetary habitability capable of sustaining chemolithotrophic life on planets where photosynthesis may never have arisen. These terrestrial discoveries have catalysed an expanded search for habitable environments on planets, exoplanets and moons to include not only surface based life but potentially vast subsurface biospheres.