The deepest part of the ocean is known as the abyssal plain. Because this area is so deep, it is difficult to explore, since it presents some unique challenges including extreme pressures. As a result, much of it has not been mapped or extensively studied, and it considered the great unknown of the Earth; some scientists believe that we may know more about Mars than we do about the abyssal plain.
Several things characterize the abyssal plain, distinguishing it from other ocean features like the continental shelf. The extreme depth is obviously the first, but this area is also very flat, with an extremely gentle slope. The flat plain is covered in millions of years of sediment washed from the continents and deposited by various sea creatures. Since it is so deep, there is no light, and oxygen saturation levels are also very low.
There are several distinct abyssal plains across the world's oceans. Each one starts at a continental rise and continues until it reaches a mid-oceanic ridge, resuming on the other side. Mid-oceanic ridges are huge underwater mountain chains marking major plate boundaries. These ridges are also the primary source of seafloor spreading, since they are slowly pulling apart. Since the continental slope and the ridges essentially form the edge of a deep bowl, some people refer to the abyssal plain as the ocean basin. Overall, it represents around 40% of the ocean floor.
While some people visualize the abyssal plain as desert-like, this characterization is far from the truth. The environment is actually teeming with life, since a multitude of organisms have adapted to the intense conditions. Organisms which live on or around this area have specialized body structures which are designed to handle the pressure, and they often look unusual and fantastic; photographs and drawings of these creatures could be mistaken for alien life.
One unusual trait which many creatures on the abyssal plain have is bioluminescence. Since there is no light on the plain, the animals make their own; some creatures use their lights like fishing lures to attract prey. Other animals are able to use chemosynthesis for energy, such as those found around hydrothermal vents. In addition to being intrinsically interesting, the abyssal plain may also provide clues to earlier life on Earth.