SEEN FROM SPACE, Earth is clearly an ocean planet, a bright blue ball swaddled in clouds of water vapor. Oceans cover 71 percent of our planet's surface and clearly are its most dominant feature. By comparison, the continents seem an afterthought, an irregular brownish-green matte in which to frame our world's greatest masterpiece. For it is the oceans, not the land, that make our planet unique, setting it apart from the dozens of dead worlds in our solar system. The oceans gave our planet life, coddled and nurtured it, and allowed it to colonize the hostile environment of the land. And it is upon the profusion of life within the oceans that all of us oxygen-breathing life forms depend for our survival.
The oceans are the cradle of life, but for most of human history this fact eluded us. That's not surprising. We are a land-dwelling, air-breathing species, separated by 360 million years of evolution from the undersea home of our distant ancestors. Without special equipment we can't see properly underwater, nor can most of us visit for more than a minute or so for lack of air. Unable to explore and participate in the undersea realm, humans regarded it as a dark and threatening place, the lair of monsters and wrathful gods for whom men and ships were favored quarry. From ancient times, sheltered seas and coastal waters surrendered fish and fostered trade, but the oceans were a frightening desert, a formidable obstacle between life-sustaining continents and islands.