The continental shelves are the relatively shallow zones immediately adjacent to almost all continent boundaries. Depths range to about 400 to 500 feet before dropping off into the continental rise and, eventually, the deep ocean floor. Continental shelves are where humans most often interact with the ocean. These areas provide the habitat for much of the seafood caught by fishermen and consumed by a worldwide population. Critically important coral reefs can exist only in the shallow depths of shelf areas.
At the deepest parts of the ocean floor, though not necessarily in the center, as might be expected, are the physical formation known as trenches. Trenches are formed where tectonic plates run together. Active plate boundaries associated with deep sea trenches are also a region where underwater volcanoes form, which eventually spawns island chains on the surface, such as Hawaii. Were it not for trenches, volcanoes and plate tectonics churning away below the sea, many of the islands around the world would not have formed.
The abyssal plains of the ocean depths are so vast as to challenge the human imagination. These flat areas that occupy the ocean floor between the foot of the continental slope and the mid-ocean ridge can be found at about 9,000 feet depth and exhibit some of the flattest, most featureless terrain to be found anywhere. Minerals essential to human survival -- salt, bromine, magnesium and others -- leach from the ocean floor into seawater, where they are eventually claimed and processed for commercial use.
There are other reasons the ocean floor is important. Sand, gravel and oil deposits are found in the near-shore areas of the continental shelf, all highly sought by humans for use in one form or another. Deeper below the water surface, scientists have long been aware of the existence of what are called manganese nodules. Iron, nickel, copper, cobalt and probably most other basic earth elements are strewn along the plains of the Pacific Ocean, locked up inside nodules that range from potato size to several feet across. A fortune is waiting for whoever first figures out how to mine at that depth.