IN 2013, SCIENTISTS were stunned to find microbes thriving deep inside volcanic rocks beneath the seafloor off the Pacific Northwest, buried under more than 870 feet of sediment. The rocks were on the flank of the volcanic rift where they were born, and they were still young and hot enough to drive intense chemical reactions with the seawater, from which the microbes derived their energy. Now, however, another team of researchers have discovered living cells inside exceedingly old, cold oceanic crust in the middle of the South Pacific. It isn’t yet clear how these new microbes are managing to survive—and...