Microorganisms in marine subsurface sediments substantially contribute to global biomass.Sediments warmer than 40°C account for roughly half the marine sediment volume, but theprocesses mediated by microbial populations in these hard-to-access environments are poorlyunderstood. We investigated microbial life in up to 1.2-kilometer-deep and up to 120°C hotsediments in the Nankai Trough subduction zone. Above 45°C, concentrations of vegetativecells drop two orders of magnitude and endospores become more than 6000 times more abundantthan vegetative cells. Methane is biologically produced and oxidized until sediments reach 80°to 85°C. In 100° to 120°C sediments, isotopic evidence and increased cell concentrationsdemonstrate the activity of acetate-degrading hyperthermophiles. Above 45°C, populated zonesalternate with zones up to 192 meters thick where microbes were undetectable