This Gall/Peters map of the world shows ocean depth using five depth contours. The coastline (the O m depth contour) is white, the 3000 m depth contour blue, the 4000 m depth contour yellow, the 5000 m depth contour green and the 6000 m depth contour red. (Compare the contour legend in the figure.)

At first sight the figure looks confusing and the amount of information overwhelming. Let us concentrate on the main features:

• The mid-ocean ridges are generally less than 3000 m deep and therefore delineated on their sides by the 3000 m depth contour; in other words, they are seen as strips of blue surrounded by yellow (the deeper water on either side of the ridges). You should be able to see such blue strips running through the centres of all oceans.
• The deep sea basins are generally between 4500 m and 5500 m deep and therefore contain the 5000 m depth contour; in other words, they are seen as areas dominated by green. Some basins are deeper than 6000 m and therefore dominated by red. You should be able to see both types of basins (those of 4500 - 5500 m depth and those deeper than 6000 m) in all oceans.

Take some time to compare the three oceans. In the Atlantic Ocean, the Mid-Atlantic Ridge (1) can be seen as a continuous mountain system extending from Greenland to the Southern Ocean. The rapid transition from green to blue indicates that it rises relatively steeply from the ocean basins.

The Indian Ocean is divided by several ridges. The Central Indian Ridge (2) extends from Arabia southwards and bifurcates in the southern hemisphere. The Southwest Indian Ridge (3) continues it to the southwest into the Atlantic Ocean, connecting it with the Mid-Atlantic Ridge. The Southeast Indian Ridge (4) continues it to the southeast into the Great Australian Bight, connecting it with the ridge system of the Pacific Ocean. This connection is broken south of the Great Australian Bight where water depths exceed 4000 m. This region is know as the Australian-Antarctic Discordance (5).
A prominent feature of the Indian Ocean is the 90º E Ridge (6), a narrow ridge that stretches in nearly perfect north-south orientation from the Bay of Bengal along the longitude 90ºE. It is very steep, as can be seen from the low amount of yellow around it.

The major ridge system in the Pacific Ocean, the East Pacific Rise (7), is located on the eastern side of the ocean. As its name implies, it is not as steep as the other ridges but rises gently; this is seen by the wide separation between blue and yellow. In the south it connects with the Pacific-Antarctic Ridge (8), which in turn provides the connection with the Indian Ocean ridge system.
The remainder of the Pacific Ocean is dominated by deep basins in the east and by seamounts and small ridge systems in the west. Among the important ones - from the point of view of the oceanic circulation - is a ridge/seamount system stretching southward from the Bering Sea, known as the Emperor Seamounts (9) in the north and the Hawaiian Ridge (10) in the south. Finally, the Izu Ridge (11) south of Japan has a major influence on the Kuroshio, the western boundary current of the North Pacific subtropics.