Abyssal plains are flat or very gently sloping areas of the ocean basin floor where rocks gradually sink into the ground because they have no supporting heat energy below them; the world's flattest and smoothest regions are caused by this effect. Abyssal plains reach depths of between 2,200 and 5,500 m (7,200 and 18,000 ft). They generally lie between the foot of a continental rise and a mid-oceanic ridge.

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 comprises chiefly dust (clay particles) blown out to sea from land, and the remains of small marine plants and animals (the plankton), which sink from the upper layer of the ocean.

Abyssal plains are less common in the Pacific than in other major ocean basins because sediments from turbidity currents are trapped in submarine trenches that border the Pacific Ocean.