Does there exist, deep within the earth's crust, a second biosphere-- composed of very primitive, thermophilic (heat-loving) bacteria, and containing more living matter than the entire surface? This idea, first proposed by the author in the early 1980s, is now supported by a growing body of evidence. The implications are astonishing: is the deep biosphere where life originated? Can Mars and other seemingly dead planets contain deep biospheres? Is there yet another--deeper, hotter--biosphere within the earth, based on silicon instead of carbon? This is the first book to explore this very controversial, intriguing theory.