The original version of this story appeared in Quanta Magazine.
Scientists have come to realize that in the soil and rocks beneath our feet there lies a vast biosphere with a global volume nearly twice that of all the world’s oceans. Little is known about these underground organisms, who represent most of the planet’s microbial mass and whose diversity may exceed that of surface-dwelling life forms. Their existence comes with a great puzzle: Researchers have often assumed that many of those subterranean realms are oxygen-deficient dead zones inhabited only by primitive microbes keeping their metabolisms at a crawl and scraping by on traces of nutrients. As those resources get depleted,
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