The seabed of the ocean harbors diverse microbial communities that are adapted to a wide range of chemical, physical, and geological conditions. These communities may extend several kilometers below the seafloor (1–4), and since the 1990s, ocean scientists have assumed that the biomass of this deep biosphere dwarfs that in the overlying ocean (1, 5). When expressed as mass units of carbon, between 10 and 30% of Earth's life is estimated to be buried as microbial biomass beneath the seabed (1, 5, 6). However, a recent study by Kallmeyer et al. (7) suggests that this fraction may be only 1%.