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.