If geography is the science of space, then few spaces are more important to humanity than the world-ocean. The world-ocean—the term preferred by geographers for the “seven seas,” which, in fact, constitute one unified geophysical system—covers 71% of Earth's surface and contains 97% of the planet's water. More than 20% of the world's petroleum is derived from offshore sources, and 95% of world trade by weight, or two thirds by value, is carried by ship. Eighty percent of the world's fish catch comes from the [Page 2065]ocean, supporting the livelihoods of 140 million people. Economists have calculated that the world-ocean provides services to humanity valued at $21 trillion per year, as opposed to only $12 trillion per year provided by land. This entry reviews some ...