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An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 meters. Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth’s surface. Abyssal plains were not recognized as distinct physiographic features of the sea floor until the late 1940s and until very recently, few have been systematically studied. 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.

Due in part to their vast size, abyssal plains are currently believed to be a major reservoir of biodiversity. The abyss also exerts significant influence upon ocean carbon cycling, dissolution of calcium carbonate and atmospheric CO₂ concentrations over timescales of 100–1000 years. The structure and function of abyssal ecosystems are strongly influenced by the rate and composition of food flux to the seafloor. Factors such as climate change, fishing practices, and ocean fertilization are expected to substantially alter primary production patterns in the euphotic zone.