For the first time, metabolically active bacterial cells have recently been quantified in deep marine sediments. In contrast to previous total cell counts that do not differentiate between active cells and inactive or dormant cells, these quantifications using oligonucleotide hybridization probes target active cells and their ribosomal (r)RNA. They demonstrate a sizable, active bacterial subsurface biosphere, and allow realistic estimates of cell-specific respiration rates and turnover times for living bacteria in this global extreme habitat. In situ activities and physiologies of these active subsurface microbiota emerge as high-priority research areas.