Earth’s subsurface is home to distinctive microbial ecosystems – often called the “deep biosphere.” Microbial activity in the terrestrial and marine deep biosphere drives global geochemical cycles, yet processes are poorly constrained. In addition, some microbial communities can persist over geological time periods despite energy-starvation, and they can revive after prolonged periods of dormancy. Unlocking the secrets of this biosphere is thus important for understanding how life survives “buried alive” and harnesses available energy sources, which has implications for the search for life elsewhere in the universe. The deep biosphere is also vulnerable and relevant to human activities such as mining, fracking, waste disposal, and carbon capture and sequestration industries. In this session, we invite presentations of the form, function, and geomicrobiology of Earth’s subsurface environments, of how the deep biosphere responds to perturbations, and of the astrobiological translation to the habitability of other planetary bodies.