A large number of survey ships have tried to locate and sound the deeps in the oceans. The soundings made by the Challenger Expedition were taken by using a fine hemp line with a weight tied on the end.

This was, of course, a very crude and old method so that the accurate measurement of the depths was not possible. But with the passage of time, a very speedy and highly accurate method was evolved.

Today echo-sounding is employed to measure the depth of the oceanic deeps. By echo-sounding either sonic or ultra-sonic vibrations are transmitted down through the water to the bottom of the ocean, which returns in the form of an echo which is electrically recorded.

When the accuracy of the highest degree is required, the ship from which measurement is to be taken is anchored. But for common purposes a continuous profile is procured from the moving ship.


The term ‘oceanic deeps’ refers to two types of depressions found on the ocean floor. The depressions are classified as ‘trenches’ and ‘troughs’. Trenches are defined as long, narrow, and deep depressions on the ocean floor, with relatively steep sides. They are usually arc- shaped.

Oceanic troughs, on the other hand, are long and relatively broader than the trenches. However, their sides have gentle slopes.

The oceanic trenches in fact represent the deepest parts of the ocean. Most of the trenches are generally found near the margins of the oceans or along the outer side of island arcs.

Most of these trenches are usually found adjacent to areas that have a great deal of volcanic and earthquake activity. They are most common on the convex side of island arcs. There are many trenches, even the deepest ones, found around the margin of the Pacific Ocean. There is one such trench close to the West Indies.


Such features are characteristic of the rim of the Pacific Ocean along the coast of South America, Middle America, and the western margin of the Pacific Ocean. The oceanic trenches are also found along the coasts of the Aleutian Islands.

It is worthwhile to bear in mind that the Challenger Deep, in the South Pacific’s Marianas Trench, is the deepest part of the ocean. In 1960, Jacques Piccard and Donald Walsh descended to the bottom of that trench which is 10,900 meters below sea level.

It is interesting to note that if Mt. Everest, the highest mountain on the earth, is placed in this trench, it would be completely drowned, and would still have 1.6 km of water above its peak.

Like the mid-oceanic ridges, the trenches appear to play a very dominant role in the geological evolution of the earth. It is the firm belief of a large number of scientists that the part of the earth’s crust that is generated at the oceanic ridges is being recycled to the earth’s interior by way of trenches.


This belief is based on the theory of plate tectonics. Even though the opinions of the scientists differ as regards the origin of oceanic trenches, it seems just possible that the compressing of the upper crust and the shearing of the crustal blocks must have simultaneously acted and contributed to the formation of the deep-sea trenches.

The major characteristic of these oceanic trenches is that they are generally asymmetrical. The slope nearer the land is much steeper than that on the side of the open ocean.

They are most commonly found near and parallel to coasts closely flanked by the young – folded mountains. Of all the oceanic troughs found on the bottom of the ocean, the Barlet Trough in the Caribbean Sea and the Waver Trough in the Molucca Sea are the most important.

Though various hypotheses have been proposed to explain the formation of the oceanic trenches, more recent work has thrown doubt on the validity of earlier interpretation of these submarine topographical features.