Fifty years of scientific ocean drilling have shown that microorganisms are widespread deep inside the ocean floor. Microbial populations exist in both organic-matter-rich and nutrient-​poor sediments (Kallmeyer et al., 2012; D’Hondt et al., 2015), in sediments that are millions of years old and are buried to over a kilometer depth (Roussel et al., 2008; Ciobanu et al., 2014; Inagaki et al., 2015), and deep inside the basaltic oceanic crust (Orcutt et al., 2011; Lever et al., 2013). In these varied environments, metabolic activity is extraordinarily low (D’Hondt et al., 2009; Hoehler and Jørgensen 2013; Lever et al. 2015a), but microbial cells remain physiologically active (Morono et al., 2011) or survive in their dormant phases (Lomstein et al., 2012). The total amount of subsurface biomass is still being debated (Hinrichs and Inagaki, 2012; Kallmeyer et al., 2012; Parkes et al., 2014), and the factors posing ultimate limits to deep life and the habitability of Earth remain to be resolved (Figure 1).

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