The Abyssal Plains
About 10 percent of the ocean floor consists of abyssal plains, which marine geologists believe are the flattest areas on Earth. The abyssal plains are found in several regions, where solid particles from rivers and from the shells of tiny marine organisms settle to the bottom to form thick, smooth layers of sediments. Farther out in the ocean, most sediment comes from the shells of tiny marine organisms. The shells drift slowly down through the water when the organisms die and accumulate on the bottom. The lower layers of the sea-floor sediments become packed hard by the weight of the sediment above them. The deepest sediments are several kilometers thick and nearly 200 million years old.
Abyssal hill regions, which cover an estimated one-fourth to one-third of the deep-sea floor, are almost as flat as the true abyssal plains, but they are underlain by mountains, ridges, and valleys. These features are buried under a heavy blanket of sediment. Here and there, the top of a particularly lofty peak protrudes from the sediment.
The most prominent feature of the deep-ocean floor is the Mid-Ocean Ridge, an enormous undersea mountain chain. When explorers first discovered evidence of the Mid-Ocean Ridge in the mid-1800's, they thought there were many separate ridges in each ocean, so they gave each part of the ridge they found a different name, such as the Mid-Atlantic Ridge, which divides the Atlantic Ocean nearly down the middle. Since then, they have learned that the Mid-Ocean Ridge is a single formation stretching about 60,000 kilometers (37,000 miles) around the globe. However, the original names for its various segments are still often used.