Regions of the deep ocean characterized by smooth ocean floor are called

The abyssal plains

Explanation:

The abyssal plains are the regions in the seafloor that are comparatively flat and extends upto hundreds of kilometer wide and a few thousands of kilometer in length. They are usually at a depth ranging from 3500 to 6000 m below the sea level.

The topography of these plains are relatively very smooth. The rate of accumulation of the sediments is very slow and they were assumed to be a stable environment that has undergone least changes. The slope of these abyssal plain is smaller than the continental shelf.

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Regions of the deep ocean characterized by smooth ocean floor are called abyssal plains.