The deep sea plains, also called abyssal plains, are a broad and nearly level area covering about two-thirds of the entire ocean floor. They extend from the base of the deep sea fans and the marginal trenches.

These plains lie at depths between 2000 and 3000 fathoms. They include flat surfaces with slopes of less than 1:1000. However, the deep sea plains are not as flat as was formerly thought.

The Albatross Expedition found from continuous echo-sounding that the bottom profile is rather rugged in detail. It was also discovered that despite the absence of steep slopes, the ocean floor is characterized by different types of topographical features.

The major characteristic of the abyssal plains is the complete absence of sediments brought by the Rivers. It certainly does not mean that the bottom is composed of naked rocks.

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On the contrary, their surface is covered with various pelagic oozes. In certain areas volcanic peaks rise steeply from these plains, and sometimes reach the surface as isolated islands.

The pelagic oozes consist of skeletal remains of innumerable Dead Sea animals and the vegetables. In the deeper parts are found the deposits of red clay which appear to be the volcanic ash deposited there by the winds blowing over the oceans.

Another characteristic feature of these plains is that those erosional processes, which produce various types of landforms on the continents, are not present there. In fact, the volcanic and tectonic activities produce the bottom relief of these plains.

Moreover, the effect of weathering and sub-aerial erosion being nil on the deep ocean floor, all the topographical features are characterized by extreme sharpness. Landforms produced by submarine volcanic activities or by folding and faulting tend to conserve their original form.

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Various research vessels with the latest observational techniques have successfully discovered various types of topographical features on the deep sea plains.

These submarine features include the oceanic ridges, trenches, seamounts and guyots etc. These landforms under the sea water are described under the respective oceans in which they occur.