Since the advent of echo-soundings, an enormous amount of information has become available regarding the main submarine features of the seas and oceans. Each of the major land masses is surrounded by a flat, gently sloping zone known as the Continental Shelf which extends from the coast out to a depth of 50–100 fathoms. The width of the Shelf varies from less than 1 mile up to several hundred miles, but 10–100 miles is usual. Outside the Shelf, the slope of the sea bed steepens and passes into the Continental Slope which descends relatively rapidly from the edge of the Shelf down to depths in the region of 2,000–3,000 fathoms. The average gradient on the Slope is normally much steeper than the gradient of the Shelf. At the foot of the Slope, the seaward gradient flattens out, sometimes abruptly, into the Ocean Basin. This is a wide, undulating, but relatively flat zone lying at 2,000–3,000 fathoms and covering most of the central parts of the major oceans.

The surface of the Shelf, which is dominantly flat, is diversified only by local banks and reefs, but the Slope is more irregular, being cut in many areas by the large marine valleys known as submarine canyons. These tend to occur in Slope areas of relatively steep gradient and generally run from the edge of the Shelf to the foot of the Slope. The canyons may reach a width of 5–10 miles, and the bottom of a canyon may reach a depth of 500 fathoms below the general level of the Slope on either side. In a few areas the Slope is also diversified by ridges and basins running more or less parallel to its trend.

The monotony of the ocean floor is diversified in many places by the presence of the Oceanic Ridges, submarine mountain ranges up to several hundred miles in width, several thousand miles in length, and rising from the level of the surrounding ocean floor to depths of only 1,500–1,000 fathoms or less. In other parts of the ocean there occur “seamounts”, isolated submarine mountains which rise from the ocean floor to within a few hundred fathoms of the surface. These seamounts may be as much as 20–30 miles across at the base, and the sides of a seamount usually have quite gentle gradients.

Another important but localised group of features lying at intermediate depths are the “continental borderlands”. These occur near to or immediately adjoining continental blocks, and include a series of broad submarine plateaux and ridges, 100–500 miles wide and lying at depths of 200–800 fathoms, together with intervening depressions. Some of these features have smooth surfaces, but others are irregular with numerous subsidiary banks and basins.

The greatest depths in the ocean are not found over wide areas of the ocean floor, but are concentrated in the Trenches which are large elongated depressions 50–150 miles wide and up to several hundred miles long. The Trenches, which occur mainly in the Pacific Ocean and are located along various parts of the periphery of the Ocean Basin, may descend to depths over 2,000 fathoms below the general level of the nearby ocean floor. The greatest depth recorded to date, the Challenger Deep (5,940 fathoms), occurs in the Mariana Trench. This deep was discovered in 1951 by H.M.S. Challenger II.