A substantial fraction of living planetary biomass consists of highly diverse microorganisms residing beneath the seafloor and soil zones. Microbial life in subsurface zones persists in extreme environments where nutrients may be scarce, the pace of life slow, and the demand on DNA repair mechanisms intense. Key questions include how the same microbial species can occupy very distant subsurface sites and what are the temperature extremes at which life cannot exist. Experiments being designed for the Deep Underground Science and Engineering Laboratory promise insights into many subsurface microbial secrets, perhaps including clues to the origins of life. © 2009 American Society for Microbiology.