able in the subsurface (Wolfe-Simon et al., 2010). This suggests that life may not be as elementally limited as postulated, and that a biosphere of arsenic-utilizing life may exist in the subsurface on Earth, other planets, moons, or asteroids. Despite these results, which question and extend our fundamental understanding of life, its constituents, evolution, and metabolic functions, some question why we should invest millions of research dollars into studying extreme environments deep underground; yet, studying these systems has many practical applications. We need to know how subsurface microbial processes affect the mobility of radioactive isotopes from buried atomic waste, and how microbial processes affect the underground storage of CO 2 . There are also technological opportunities hidden in the deep subsurface. Subsurface microbiota may be useful for the in situ processing of underground ore deposits, cleaning up of polluted groundwaters, and, in the spirit of Parkes et al., we may be able to use the microbially mediated formation of H 2 (or other gases) as a way of producing clean energy. Paving the way for these discoveries are advances in deepdrilling technology, fi eld-sterile sampling, microanalyses, metagenomics, transcriptomics, proteomics, and bioinformatics. These will enable geomicrobiologists to study microbial diversities, capabilities, and activities on the scale of entire subsurface ecosystems, and link microbial factors to geochemical and geological factors.