Abyssal plain

An abyssal plain is an underwater plain on the deep ocean floor. It is usually found 3,000 metres (9,800 ft) and 6,000 metres (20,000 ft) below the surface of the water. Abyssal plains cover more than 50% of the Earth's surface. They are among the flattest and smoothest places on Earth.
Abyssal plains are believed to have lots of biodiversity. They are also play an important role in ocean carbon cycling, dissolution of calcium carbonate, and atmospheric CO2 concentrations over hundreds or thousand years. Abyssal plains also have hydrothermal vents and cold seeps.