1 Chivian D. et al. Environmental genomics reveals a single-species ecosystem deep within Earth. Science. 322, 275–78 (2008).

2 Ruff S. E. et al. Hydrogen and dark oxygen drive microbial productivity in diverse groundwater ecosystems. Nat. Commun. 14, 3194 (2023); Kraft B. et al. Oxygen and nitrogen production by an ammonia-oxidizing archaeon. Science. 375, 97–100 (2022).

3 LaRowe D. E. et al. Temperature and volume of global marine sediments. Geology. 45, 275–78 (2017).

4 Karner M. B. et al. Archaeal dominance in the mesopelagic zone of the Pacific Ocean. Nature. 409, 507–10 (2001); Parkes R. J. et al. Recent studies on bacterial populations and processes in subseafloor sediments: A review. Hydrogeol. J. 8, 11–28 (2000).

5 Kallmeyer J. et al. Global distribution of microbial abundance and biomass in subseafloor sediment. Proc. Natl. Acad. Sci. USA. 109, 16213–16 (2012).

6 Magnabosco C. et al. The biomass and biodiversity of the continental subsurface. Nat. Geosci. 11, 707–17 (2018).

7 Lloyd K. G. et al. Spatial structure and activity of sedimentary microbial communities underlying a Beggiatoa spp. mat in a Gulf of Mexico hydrocarbon seep. PLOS One. 5, e8738 (2010).