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Abyssal plains are flat or very gently sloping areas of the deep ocean basin floor. They are among the Earth's flattest and smoothest regions and the least explored. Abyssal plains cover approximately 40% of the ocean floor and reach depths 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.

The abyssal plain is formed when the lower crust (sima), melted and pushed up by the up-welling mantle, reaches the surface at mid-ocean ridges and forms new oceanic crust. This new oceanic crust is mostly basalt and has a rugged topography. The roughness of this topography is a function of the rate at which the mid-ocean ridge is spreading (the spreading rate). Magnitudes of spreading rates vary quite significantly, and are generally broken down into 3 rates (fast, medium and slow). Typical values for fast-spreading ridges are >100 mm/yr, whilst medium-spreading rates are ~60 mm/yr, and slow-spreading ridges are typically <20 mm/yr. Studies have shown that the slower the spreading rate, the rougher the new oceanic crust will be, and vice versa. It is thought this is due to faulting at the mid-ocean ridge when the new oceanic crust was formed. This oceanic crust eventually becomes overlain with sediments, producing the flat appearance.

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, known as Pelagic sediment. The sediment deposition rate in remote areas is estimated at two to three centimetres per thousand years. In some areas of the plains manganese nodules are common with significant varying concentrations of metals, including iron, nickel, cobalt, and copper. These nodules provide a significant resource for future mining ventures.

Sediment covered 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.

This deep seafloor, between 4 and 6 km under the surface, is without comparison the largest habitat on earth, covering 60% of the earth's surface.

• Labrador Plain
• Irminger Plain
• Iceland Plain
• Norwegian Plain
• Cape Verde Plain
• Sierra Leone Plain
• Guinea Plain
• Angola Plain
• Cape Plain
• Argentine Plain
• Brasil Plain
• Guayana Plain
• Mexico Plain

• Agulhas Plain
• Natal Plain
• Madagaskar Plain
• Somalian Plain
• Arabian Plain
• Middle-Indian Plain
• Andamanian Plain
• Wharton Plain
• Perth's Plain
• Crozet's Plain
• Atlantic-Indian Plain
• North Australian Plain
• South Australian Plain

• Celebes' Plain
• South Chinese Plain
• West Carolinian Plain
• East Carolinian Plain
• Coral sea Plain
• Tasmani Plain
• Southern Fiji Plain
• Northern Fiji Plain
• Melanesian Plain
• East Mariana Plain
• Northwestern Pacific Plain
• Japanese Plain
• Kurillian Plain
• Middle Pacific Plain
• Southwestern Pacific Plain
• Southeastern Pacific Plain
• Chile Plain
• Peru Plain
• Guatemala Plain