From the window of an airplane or the deck of a ship, the ocean appears to be almost featureless. Beneath the rolling blue waves, however, is a seemingly endless world of mountains, valleys, and diverse organisms engaged in life-and-death struggles. Scientists have learned much about the features and life forms of this vast realm.

The world ocean is divided into three major bodies of water: the Pacific, Atlantic, and Indian oceans. The Pacific and Atlantic are further subdivided into North and South Pacific and North and South Atlantic. The Pacific, Atlantic, and Indian oceans are connected at their southern boundaries by the Antarctic, or Southern, Ocean. Lastly, the Arctic Ocean, which many oceanographers consider to be part of the Atlantic, lies at the northern end of the Earth. Together, the seven ocean divisions--the “Seven Seas”--cover more than 70 percent of Earth's surface and contain about 97 percent of all the water on the planet.

The Pacific is the largest of the three main oceans, with a surface area of 181 million square kilometers (70 million square miles) and an average depth of 3,940 meters (12,900 feet). The basin of the Pacific (the rocky depression that forms the bottom and sides of the ocean) is the most geologically active of the ocean basins. It contains numerous volcanic vents, high seamounts (isolated underwater mountains), and deep trenches (long, narrow valleys).

The Atlantic Ocean has a surface area of 94 million square kilometers (36 million square miles) and an average depth of 3,580 meters (11,700 feet). The floor of the Atlantic basin is dominated by a mountain chain called the Mid-Atlantic Ridge, which runs down the basin's central part.

The Indian Ocean, which lies between Africa, Indonesia, and Australia, is the smallest of the major oceans. It has an area of 74 million square kilometers (29 million square miles) and an average depth of 3,840 meters (12,600 feet).