Deep drilling of marine sediments and oceanic crust offers a unique opportunity to explore how life persists and evolves in the Earth's deepest subsurface ecosystems. Resource availability deep beneath the seafloor may impose constraints on microbial growth and dispersal patterns that differ greatly from the surface world. Processes that mediate microbial evolution and diversity may also be very different in these habitats. Communities in parts of the deep subsurface may resemble primordial microbial ecosystems, and may serve as analogues of life on other planets that have, or once had, water. In short, the deep biosphere is one of the least explored biomes on Earth and deserves intense exploration.