Subsurface advective flow unveils the architecture of Earth’s crustal biosphere

The subsurface biosphere is one of Earth’s largest microbial reservoirs, yet its spatial extent remains poorly constrained due to limited direct access. Here we propose using the integrated environmental readouts encoded in deep subsurface fluids to shift from simply mapping where life can be detected to constraining where life can exist. Deeply-sourced advective fluids provide an integrated signal of subsurface microbial processes, recording the combined effects of heat flow, water–rock interaction, volatile inputs, and microbial activity along fluid pathways. By coupling surface measurements of fluid temperature, geochemistry, and volatile provenance embedded in diverse tectonic settings, we can define the maximum theoretical depth of the habitable crust and constrain the degree of surface overprint. This framework enables comparisons across geological settings and scales that are otherwise inaccessible, linking subsurface habitability to crustal structure and dynamics. It provides a quantitative basis to evaluate the extent of Earth’s subsurface biosphere from the surface, and to extend these principles to other planetary bodies.