Areas of the ocean floor that are distant from the oceanic ridges become sunken. These regions are the world's smoothest, flattest areas and are called abyssal plains.

The continents extend out into shallow ocean water; these areas are called continental shelves or margins. Where these shelves end, the ocean floor drops down to the abyssal plains. Perhaps you have heard the phrase “falling into the abyss.” The words “abyss” or “abyssal” mean “too deep to measure.” As you might guess, the abyssal plains are dark and murky places.