Abyssal plain

An abyssal plain is a flat expanse of the deep ocean floor, usually lying between 3,000 and 6,000 meters deep, extending from the foot of a continental rise out to a mid-ocean ridge and together covering more than half of the Earth's surface. Abyssal plains rank among the flattest, smoothest and least explored parts of the planet, and stand as one of the key structural elements of an oceanic basin alongside the elevated mid-ocean ridge and the abyssal hills that flank it.