The uppermost portion of the photic zone, where there is adequate light to support photosynthesis by phytoplankton and plants, is referred to as the euphotic zone (also referred to as the epipelagic zone, or surface zone). The photic zone can be subdivided into two different vertical regions. The stratum of the water column up till which sunlight penetrates is referred to as the photic zone. Photosynthesis in turn requires energy from sunlight to drive the chemical reactions that produce organic carbon. Nearly all life forms in the ocean depend on the photosynthetic activities of phytoplankton and other marine plants to convert carbon dioxide into organic carbon, which is the basic building block of organic matter. The ocean can be conceptualized as being divided into various zones, depending on depth, and presence or absence of sunlight. The sonar system uses phase and amplitude bottom detection, with an accuracy of better than 0.2% of water depth (this is an error of about 22 meters at this depth). On 1 June 2009, sonar mapping of the Challenger Deep by the Simrad EM120 multibeam sonar bathymetry system aboard the R/V Kilo Moana indicated a maximum depth of 10,971 meters (6.817 miles). Nereus reached a depth of 10,902 meters at the Challenger Deep on May 31, 2009. Nereus, a hybrid remotely operated vehicle (HROV) of the Woods Hole Oceanographic Institution, is the only vehicle capable of exploring ocean depths beyond 7000 meters. The Japanese remote operated vehicle (ROV) Kaiko became the second vessel to reach the bottom of the Challenger Deep in March 1995. At this great depth a small flounder-like fish was seen moving away from the spotlight of the bathyscaphe. In 1960, Don Walsh and Jacques Piccard descended to the bottom of the Challenger Deep in the Trieste bathyscaphe. The reported depth was 4,475 fathoms (8184 meters) based on two separate soundings. The depression is named after HMS Challenger, whose researchers made the first recordings of its depth on 23 March 1875 at station 225. The Challenger Deep is the deepest surveyed point of all of Earth's oceans it is located at the southern end of the Mariana Trench near the Mariana Islands group. van Andel, first witnessed dense chemosynthetic clam beds from the submersible DSV Alvin on February 17, 1977, after their unanticipated discovery using a remote camera sled two days before. The first discovery of any deep-sea chemosynthetic community including higher animals was unexpectedly made at hydrothermal vents in the eastern Pacific Ocean during geological explorations (Corliss et al., 1979). In the 1870s Sir Charles Wyville Thomson and colleagues aboard the Challenger expedition discovered many deep-sea creatures of widely varying types. Prior to the 19th century scientists assumed life was sparse in the deep ocean. The three main sources of energy and nutrients for deep sea communities are marine snow, whale falls, and chemosynthesis at hydrothermal vents and cold seeps. Since the 19th century however, research has demonstrated that significant biodiversity exists in the deep sea. Because of the unique challenges (particularly the high barometric pressure, extremes of temperature and absence of light), it was long believed that little life existed in this hostile environment. Deep sea communities remain largely unexplored, due to the technological and logistical challenges and expense involved in visiting this remote biome. A deep-sea community is any community of organisms associated by a shared habitat in the deep sea.
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