The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline, at a depth of 1000 fathoms (1800 m) or more. Little or no light penetrates this area of the ocean, and most of its organisms rely on falling organic matter produced in the photic zone for subsistence. For this reason scientists assumed life would be sparse in the deep ocean, but virtually every probe has revealed that, on the contrary, life is abundant in the deep ocean.
From the time of Pliny until the expedition in the ship Challenger between 1872 and 1876 to prove Pliny wrong; its deep-sea dredges and trawls brought up living things from all depths that could be reached. Perhaps one day man will be more like aqua man, and roam the ocean depths with the fish creatures alike. Yet even in the twentieth century scientists continued to imagine that life at great depth was insubstantial, or somehow inconsequential. The eternal dark, the almost inconceivable pressure, and the extreme cold that exist below one thousand meters were, they thought, so forbidding as to have all but extinguished life. The reverse is in fact true....(Below 200 meters) lies the largest habitat on earth.
In 1960 the Bathyscaphe Trieste descended to the bottom of the Marianas Trench near Guam, at 35,798 feet (10,911 meters), the deepest spot on earth. If Mount Everest were submerged there, its peak would be more than a mile beneath the surface. At this great depth a small flounder-like fish was seen moving away from the bathyscaphe's spotlight. The Japanese research submersible Kaiko was the only other vessel capable of reaching this depth, but it was lost at sea in 2003.
More is known about the moon than the deepest parts of the ocean. Until the late 1970s little was known about the possibility of life on the deep ocean floor but the discovery of thriving colonies of shrimp and other organisms around hydrothermal vents changed that. Before the discovery of the undersea vents, all life was thought to be driven by the sun. But these organisms get their nutrients from the earth's mineral deposits directly. These organisms thrive in completely lightless and anaerobic environments, in highly saline water that may reach 300 °F (150 °C), drawing their sustainance from hydrogen sulfide, which is highly toxic to all terrestrial life. The revolutionary discovery that life can exist without oxygen or light significantly increases the chance of there being life elsewhere in the universe. Scientists now speculate that Europa, one of Jupiter's moons, may have conditions that could support life beneath its surface which is speculated to be a liquid ocean beneath the icy crust.
Regions below the epipelagic are divided into further zones, beginning with the mesopelagic which spans from 200 to 1000 below sea level, where a little light penetrates while still being insufficient for primary production. Below this zone the deep sea proper begins, consisting of the aphotic bathypelagic, abyssopelagic and hadopelagic. Food consists of falling organic matter known as 'marine snow' and carcasses derived from the productive zone above, and is scarce both in terms of spatial and temporal distribution.
Instead of relying on gas for their buoyancy, many species have jelly-like flesh consisting mostly of glycosaminoglycans, which has very low density. It is also common among deep water squid to combine the gelatinous tissue with a flotation chamber filled with a coelomic fluid made up of the metabolic waste product ammonium chloride, which is lighter than the surrounding water.
The midwater fish have special adaptations to cope with these conditions—they are small, usually being under 25 centimetres (10 in); they have slow metabolisms and unspecialized diets, preferring to sit and wait for food rather than waste energy searching for it. They have elongated bodies with weak, watery muscles and skeletal structures. They often have extendable, hinged jaws with recurved teeth. Because of the sparse distribution and lack of light, finding a partner with which to breed is difficult, and many organisms are hermaphroditic.
Because light is so scarce, fish often have larger than normal, tubular eyes with only rod cells. Their upward field of vision allows them to seek out the silhouette of possible prey. Prey fish however also have adaptations to cope with predation. These adaptations are mainly concerned with reduction of silhouette, a form of camouflage. The two main methods by which this is achieved are reduction in the area of their shadow by lateral compression of the body, and counter illumination via bioluminescence. This is achieved by production of light from ventral photophores, which tend to produce such light intensity to render the underside of the fish of similar appearance to the background light. For more sensitive vision in low light, some fish have a retroreflector behind the retina. Flashlight fish have this plus photophores, which combination they use to detect eyeshine in other fish (see Tapetum lucidum).
It is important to realize that organisms in the deep sea are almost entirely reliant upon sinking living and dead organic matter which falls at approximately 100 meters per day. In addition to this, only about 1-3% of the production from the surface reaches the sea bed mostly in the form of marine snow - as mentioned above. Larger food falls, such as whale carcasses, also occur and studies have shown that these may happen more often than currently believed. There are lots of scavengers that feed primarily or entirely upon large food falls and the distance between whale carcasses is estimated to only be 8 kilometers. In addition, there are a number of filter feeders that feed upon organic particles using tentacles, such as Freyella elegans.
Marine bacteriophages play an important role in cycling nutrients in deep sea sediments. They are extremely abundant (between 5x1012 and 1x1013 phages per square meter) in sediments around the world. 
There are a number of species that do not primarily rely upon dissolved organic matter for their food and these are found at hydrothermal vents. One example is the symbiotic relationship between the tube worm Riftia and chemosynthetic bacteria. It is this chemosynthesis that supports the complex communities that can be found around hydrothermal vents. These complex communities are one of the only ecosystems on the planet that do not rely upon sunlight for photosynthesis.
The deep sea is an environment totally inhospitable to humankind, and it should come as no surprise that it represents one of the least explored areas on Earth. Pressures even in the mesopelagic become too great for traditional exploration methods, demanding alternative approaches for deep sea research. Baited camera stations, small manned submersibles and ROVs (remotely operated vehicles) are three methods utilized to explore the ocean's depths. Because of the difficulty and cost of exploring this zone, current knowledge is limited. Pressure increases at approximately one atmosphere for every 10 meters meaning that some areas of the deep sea can reach pressures of above 1,000 atmospheres. This not only makes great depths very difficult to reach without mechanical aids, but also provides a significant difficulty when attempting to study any organisms that may live in these areas as their cell chemistry will be adapted to such vast pressures. If any fish or organisms from this depth were brought to the surface to be studied under laboratory conditions, the atmospheric pressure (or lack thereof) would cause them to expand or even explode.