This is a list of submarine topographical features, oceanic landforms and topographic elements. Abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains are among the flattest, smoothest and least explored regions on Earth. Abyssal plains are key geologic elements of oceanic basins (the other elements being an elevated mid-ocean ridge and flanking abyssal hills). In addition to these elements, active oceanic basins (those that are associated with a moving plate tectonic boundary) also typically include an oceanic trench and a subduction zone. Abyssal plains cover more than 33% of the ocean floor (about 23% of Earth's surface), but they are poorly preserved in the sedimentary record because they tend to be consumed by the subduction process. The abyssal plain is formed when the lower oceanic crust is melted and forced upwards by the asthenosphere layer of the upper mantle. As this basaltic material reaches the surface at mid-ocean ridges, it forms new oceanic crust. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is deposited from turbidity currents that have been channeled from the continental margins along submarine canyons down into deeper water. The remainder of the sediment is composed chiefly of pelagic sediments. Use of a continuously recording fathometer enabled Tolstoy & Ewing in the summer of 1947 to identify and describe the first abyssal plain. This plain, located to the south of Newfoundland, is now known as the Sohm Abyssal Plain. Following this discovery many other examples were found in all the oceans. List of abyssal plains and oceanic basins Following is a list of named abyssal plains and oceanic basins: Oceanic trenches Oceanic trenches are long, narrow topographic depressions of the seabed. They are the deepest parts of the ocean floor, and they define one of the most important natural boundaries on the Earth's solid surface: the one between two lithospheric plates. Trenches are a distinctive morphological feature of plate boundaries. Trenches are found in all oceans with the exception of the Arctic Ocean and they are most common in the North and South Pacific Oceans. There are three types of lithospheric plate boundaries: 1.) divergent (where lithosphere and oceanic crust is created at