Deep within the earth's crust there exists a second biosphere, composed of very primitive heat-loving bacteria and containing perhaps more living matter than is present on the earth's entire surface. That is the astounding premise of this new book by Thomas Gold, one of the twentieth century's most distinguished scientists and a man with a long history of proposing seemingly preposterous theories that later prove correct. Gold joins the deep hot biosphere argument to another, perhaps even more controversial theory for which he has marshalled evidence: that so-called fossil fuels originate not from compressed biological matter at all but from deep within the earth, present there since the planet's formation, long before our oxygen-rich surface biosphere came into existence.