The sea covers seven-tenths of the Earth, but we have mapped only a small percentage of it. The sea contains millions of species of animals and plants, but we have identified only a few thousand of them. The sea controls our planet's climate, but we do not really understand how. The sea is still the frontier, and yet it seems so familiar that we sometimes forget how little we know about it. Just as we are poised on the verge of exploiting the sea...