The global ocean is Earth's largest biome, which extends into the sediments and igneous crust below the oceans. The abundance of microbial life beneath the sea floor is at least comparable to that in the oceans (1), but this biome remains poorly understood. The ramifications of a massive buried biosphere are important on a global scale, with sub–sea-floor microbes playing a crucial role in carbon sequestration, element cycles, and Earth's evolution, and likely encompassing staggering metabolic and genetic diversity. On page 420 of this issue, Inagaki et al. (2) report that even at almost 2.5 km beneath the sea floor, microbial life is not only present and compositionally distinct from that in shallower sediments, but also producing methane.