The ocean floor off the continental shelf is known as the abyssal plain. The abyssal plain has a depth between 2,200 and 5,500 m (7,200 and 18,000 ft) and covers about 40% of the ocean floor. These areas are among the flattest and least explored on the Earth's surface. Less than one tenth of 1% of the abyssal plain has been explored by man, primarily using deep submarine robots.
The bottom of the ocean is flat because of sediments constantly building up there at an even rate. These sediments fall into three types: siliceous oozes (from silica shells), calcareous oozes (from calcite shells), and red clays (from windblown sand and micrometeorites). The sediments accumulate very slowly, just a few centimeters per millennium.
Life on the ocean floor is very sparse, making it similar to a vast desert. Marine organisms prefer shallow waters, where energy from photosynthesis is abundant, providing the foundation of the food chain. There are several oases on the ocean floor: hydrothermal vents, cold seeps, and whale falls. Hydrothermal vents release minerals rich in sulfides, which can be processed by chemotrophic bacteria acting as a keystone for small ecosystems. In cold seeps, a methane-filled brine leaks from cracks in the sea floor, also providing energy for bacteria. A whale fall is a whale carcass that falls from above. Because scavengers are so scarce on the ocean floor, it may take decades to centuries for a whale to be consumed. Scientists estimate the frequency of whale falls at approximately one every 25 km (15 mi).
The ocean floor is populated by bottom feeders that look somewhat like blobs. The blobfish, first discovered off the coast of Australia, is an example. Because of the extreme pressure, these fish have evolved a gelatinous texture of flesh with a density slightly less than sea water. This lets them float just above the surface without expending effort on swimming. Many bottom feeders have an underslung jaw, letting them scoop up nutrient-filled ooze from the ocean floor.
