Broad takes us on breathtaking dives and expeditions - to the Azores, to the Titanic, to hot springs teeming with bizarre life, to icy fissures aswarm with gulper eels, vampire squids, and gelatinous beasts longer than a city bus. We meet legendary explorers at the forefront of deep research and go with them as they probe the ancient mysteries of the deep.

This universe below encompasses the vast majority of the Earth's habitable space and nurtures perhaps ten times as many species of life as are known on land. Broad shows that the abyss also holds millions of humanity's lost artworks and treasures - more than all the world's museums combined. Yet, remarkably, human eyes up to now have glimpsed perhaps only a billionth of this unfamiliar realm, a place of crushing pressure and eternal darkness.

Drawing on extensive fieldwork, including hundreds of talks and interviews, Broad takes us to the cutting edge of the exploratory surge and reveals how it is powered by a wave of once-secret technologies. At a cost of untold billions, the United States, the Soviet Union, France, and other cold-war contestants forged these marvels to spy and fight and plunder the deep.

Today, these wonders and the people who made and ran them are catalyzing and unprecedented speedup in civilian efforts to illuminate the inky depths.

Broad shows how the rush into the deep is revealing not only great mysteries and riches but great dangers as well, including the deadly radioactive debris of the cold war. Deep pollution, mining, and fishing threaten this frontier with ecological upset and species extinction. We will either destroy the sea through ignorance or save it, and ourselves, with the kinds of knowledge we are now gaining in the exploratory speedup.