This book sets forth a set of truly controversial and astonishing theories: First, it proposes that below the surface of the earth is a biosphere of greater mass and volume than the biosphere the total sum of living things on our planet's continents and in its oceans. Second, it proposes that the inhabitants of this subterranean biosphere are not plants or animals as we know them, but heat-loving bacteria that survive on a diet consisting solely of hydrocarbons that is, natural gas and petroleum. And third and perhaps most heretically, the book advances the stunning idea that most hydrocarbons on Earth are not the byproduct of biological debris ("fossil fuels"), but were a common constituent of the materials from which the earth itself was formed some 4.5 billion years ago.
The implications are astounding. The theory proposes answers to often-asked questions: Is the deep hot biosphere where life originated, and do Mars and other seemingly barren planets contain deep biospheres? Even more provocatively, is it possible that there is an enormous store of hydrocarbons upwelling from deep within the earth that can provide us with abundant supplies of gas and petroleum?
However far-fetched these ideas seem, they are supported by a growing body of evidence, and by the indisputable stature and seriousness Gold brings to any scientific debate. In this book we see a brilliant and boldly original thinker, increasingly a rarity in modern science, as he develops potentially revolutionary ideas about how our world works.

1 Our Garden of Eden.- The Narrow Window for Surface Life.- Chemical Energy for Subsurface Life.- A Preview of This Book.- 2 Life at the Borders.- Energy Deep in the Earth.- The Ecology of Deep-Ocean Vent Life.- Other Borderland Ecologies.- Deep Is Desirable.- Beneath the Borderlands.- 3 The Deep-Earth Gas Theory.- The Origin of Petroleum: Two Conflicting Theories.- Five Assumptions Underlying the Deep-Earth Gas Theory.- 4 Evidence for Deep-Earth Gas.- Petroleum Reservoirs That Refill.- Clues in the Carbonate Record.- The Association of Helium with Hydrocarbons.- 5 Resolving the Petroleum Paradox.- The Deep Hot Biosphere Solution.- Biological Molecules in Non-Biological Petroleum.- The Upwelling Theory of Coal Formation.- Evidence for the Upwelling Theory.- An Exemption for Peat.- 6 The Siljan Experiment.- Drilling in Swedish Granite.- Magnetite and Microbial Geology.- 7 Extending the Theory.- The Origin of Diamonds.- A New Explanation for Concentrated Metal Deposits.- 8 Rethinking Earthquakes.- Mud Volcanoes.- A Challenge to Earthquake Theory.- Eyewitness Accounts.- Earthquake Spots and Earth Mounds.- Upwelling Deep Gas as the Cause of Earthquakes.- 9 The Origin of Life.- The Habitability of Surface and Subsurface Realms.- The Enhanced Probability for Life's Origin.- Darwin's Dilemma.- 10 What Next?.- Microbial Investigations.- Prospects for Extraterrestrial Surface Life.- Deepening the Search for Extraterrestrial Life.- Independent Beginnings or Panspermia?.- Afterword to the Paperback Edition.- Notes.- Acknowledgments.