What do you call a smooth, nearly flat region of the deep ocean floor?

A smooth, nearly flat region of the deep ocean floor is called an abyssal plain. These plains are often found at depths ranging from about 3,000 to 6,000 meters below sea level and cover more than 50% of the Earth's surface. The flatness of abyssal plains results from the accumulation of sediments over time, which can include mud, sand, and the remains of marine organisms. This sedimentation contributes to the smooth, featureless landscape that characterizes these regions.

In contrast, the continental shelf is the submerged part of a continent, which is generally shallow compared to the deep ocean and features a sloping profile. A seamount refers to underwater mountains that rise from the ocean floor but do not reach the water's surface, often characterized by rugged terrain rather than flatness. An ocean ridge, such as the Mid-Atlantic Ridge, is another prominent feature of the ocean floor, known for its volcanic activity and elevated terrain, rather than smoothness. Thus, the defining characteristic of an abyssal plain is its extensive flatness and depth, distinguishing it from other oceanic features.