The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline, at a depth of 1000 fathoms (1828 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 vessel capable of reaching this depth, and it was lost in 2003.
We know more 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 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 (149 °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.