The oceans—great interconnected bodies of salt water—cover 70.8 percent of the Earth's surface, a total of 139,782,000 square miles (363,433,200 square kilometers). They contain 97 percent of all the water on the Earth, a total volume of 323,860,000 cubic miles (1,327,826,000 cubic kilometers).

Oceanography, the science of the oceans, officially started in the 1870s when the British ship Challenger began its career of oceanic exploration. Another research vessel, the Meteor, equipped with electronic equipment, began exploration in 1925 and discovered great mountains and trenches on the sea floor. Only since the 1930s have people entered the deeper regions, and the deepest spot—the Mariana Trench in the Pacific Ocean—was not reached until 1960. Although new information...