This part of the ocean floor is more than 3 miles (5 kilometers) deep. A plain is a relatively flat expanse of ground.
Abyssal plains are wide, flat areas of the ocean floor on either side of the mid-ocean ridge. Sediment covers the floor of abyssal plains, deposited there by particle-filled underwater currents.
The rock under the sediment actually has a variety of hills and depressions. The sediment simply fills in the irregularities, making it appear flat.
Abyssal plains generally stretch for about 150 miles (240 kilometers) between a continental rise and the mid-ocean ridge.