Editing Underwater environment

{{Short description|Aquatic or submarine environment}}
{{Use dmy dates|date=December 2024}}
{{Use British English|date=December 2024}}
{{Redirect|Underwater}}
[[File:Great Barrier Reef snorkeling 18.jpg|thumb|250px|[[Great Barrier Reef]], Australia]]
An '''underwater environment''' is a environment of, and immersed in, liquid [[water]] in a natural or artificial feature (called a [[Water|body of water]]), such as an [[ocean]], [[sea]], [[lake]], [[pond]], [[reservoir]], [[river]], [[canal]], or [[aquifer]]. Some characteristics of the underwater environment are universal, but many depend on the local situation.

Liquid water has been present on Earth for most of the [[History of Earth|history of the planet]]. The underwater environment is thought to be the place of the [[origin of life]] on Earth, and it remains the ecological region most critical to the support of life and the [[natural habitat]] of the majority of living organisms. Several branches of science are dedicated to the study of this environment or specific parts or aspects of it.

A number of human activities are conducted in the more accessible parts of the underwater environment. These include research, underwater diving for work or recreation, and underwater warfare with submarines. It is hostile to humans in many ways and often inaccessible, and therefore relatively little explored.

==Extent==
[[File:BlueMarble-2001-2002.jpg|thumb|The world ocean is the most visible part of Earth from space.]]
Three quarters of the planet [[Earth]] are covered by water. Most of the planet's solid surface is [[abyssal plain]], at depths between {{convert|4000|and|5500|m|ft}} below the surface of the oceans. The solid surface location on the planet closest to the center of the geoid is the [[Challenger Deep]], located in the [[Mariana Trench]] at a depth of {{convert|10,924|m|ft}}. There is a smaller part of the surface covered by bodies of fresh water and a large volume of underground water in aquifers. The underwater environment is hostile to humans in many ways and therefore little explored. It can be mapped by [[sonar]], or more directly explored via manned, remotely operated, or autonomous [[submersibles]]. The ocean floors have been surveyed via sonar to at least a coarse resolution; particularly-strategic areas have been mapped in detail, to assist in navigating and detecting submarines, though the resulting maps may be classified.{{citation needed|date=June 2016}}

===Oceans and seas===
{{main|Ocean|Sea}}
[[File:Clouds over the Atlantic Ocean.jpg|thumb|Clouds over the Atlantic Ocean]]
An ocean is a body of [[water]] that composes much of a [[planet]]'s [[hydrosphere]].<ref name="pri.ocean">{{cite web |url=http://wordnetweb.princeton.edu/perl/webwn?s=ocean |title=WordNet Search — ocean |publisher=Princeton University |access-date=February 21, 2012 }}</ref> On [[Earth]], an ocean is one of the major conventional divisions of the [[World Ocean]]. These are, in descending order by area, the [[Pacific Ocean|Pacific]], [[Atlantic Ocean|Atlantic]], [[Indian Ocean|Indian]], [[Southern Ocean|Southern]] (Antarctic), and [[Arctic Ocean|Arctic]] Oceans.<ref name="oed.130201">{{cite web |url=http://www.oed.com/view/Entry/130201?redirectedFrom=ocean#eid |title=ocean, n |publisher=Oxford English Dictionary |access-date=February 5, 2012 }}</ref><ref name="mw.ocean">{{cite web |url=http://www.merriam-webster.com/dictionary/ocean |title=ocean |publisher=Merriam-Webster |access-date=February 6, 2012 }}</ref> The word "ocean" is often used interchangeably with "sea" in [[American English]]. Strictly speaking, a ''[[sea]]'' is a body of water (generally a division of the world ocean) partly or fully enclosed by land,<ref name="pri.sea">{{cite web |url=http://wordnetweb.princeton.edu/perl/webwn?s=sea |title=WordNet Search — sea |publisher=Princeton University |access-date=February 21, 2012 }}</ref> though "'''the sea'''" refers also to the oceans.

[[Saline water]] covers approximately {{convert|361,000,000|km2|abbr=on}} and is customarily divided into several principal oceans and smaller seas, with the ocean covering approximately 71% of Earth's surface and 90% of the Earth's [[biosphere]].<ref name="noaa.gov">{{cite web |title=NOAA – National Oceanic and Atmospheric Administration – Ocean |url=http://www.noaa.gov/ocean.html |publisher=Noaa.gov |access-date=2012-11-08 }}</ref> The ocean contains 97% of Earth's water, and [[Oceanography|oceanographers]] have stated that less than 100% of the World Ocean has been explored.<ref name="noaa.gov" /> The total volume is approximately 1.35 billion cubic kilometers (320 million cu mi) with an average depth of nearly {{convert|3700|m|ft|sp=us}}.<ref name="hypertextbook2003">{{cite web |last=Qadri |first=Syed |title=Volume of Earth's Oceans |work=The Physics Factbook |date=2003 |url=http://hypertextbook.com/facts/2001/SyedQadri.shtml |access-date=2007-06-07 }}</ref><ref name="Charette_Smith_Ocean-depth">{{cite journal |last1=Charette |first1=Matthew |last2=Smith |first2=Walter H. F. |title=The volume of Earth's ocean |journal=Oceanography |date=2010 |volume=23 |issue=2 |pages=112–114 |doi=10.5670/oceanog.2010.51 |doi-access=free |hdl=1912/3862 |hdl-access=free }}</ref><ref name="NOAA" >{{cite web|title=Volumes of the World's Oceans from ETOPO1 |url=https://www.ncei.noaa.gov/products/etopo-global-relief-model |publisher=[[National Oceanic and Atmospheric Administration|NOAA]] |access-date=2015-03-07 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20150311032757/http://ngdc.noaa.gov/mgg/global/etopo1_ocean_volumes.html |archive-date=2015-03-11 }}</ref>

===Lakes, ponds, and rivers===
{{main|Lake|Pond|River}}
[[File:Река Самур.jpg|thumb|[[Samur River]] in [[Azerbaijan]] – In the natural landscape]]
A lake is an area filled with water, localized in a [[Depression (geology)|basin]], that is surrounded by land, apart from any [[river]] or other outlet that serves to feed or drain the lake.<ref name="Purcell"/> Lakes lie on [[land]] and are not part of the [[World Ocean|ocean]], and therefore are distinct from [[lagoon]]s, and are also larger and deeper than [[pond]]s, though there are no official or scientific definitions.<ref name="Dictionary" /> Lakes can be contrasted with [[river]]s or [[stream]]s, which are usually flowing. Most lakes are fed and drained by rivers and streams. Natural lakes are generally found in mountainous areas, [[rift zone]]s, and areas with ongoing [[glacier|glaciation]]. Other lakes are found in [[endorheic basin]]s or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the [[Last Glacial Period|last ice age]]. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them. Many lakes are [[controlled lake|artificial]] and are constructed for industrial or agricultural use, for [[hydro-electric power]] generation or domestic water supply, or for aesthetic, recreational purposes, or other activities.

A pond is an area filled with water, either natural or artificial, that is smaller than a [[lake]].<ref name="Webster" /> It may arise naturally in floodplains as part of a river system, or be a somewhat isolated depression (such as a [[Kettle (landform)|kettle]], [[vernal pool]], or [[prairie pothole]]). It may contain shallow water with marsh and aquatic plants and animals.<ref name="Clegg 1986" /> Ponds are frequently man-made or expanded beyond their original depth and bounds. Among their many uses, ponds provide water for agriculture and livestock, aid in habitat restoration, serve as fish hatcheries, are components of landscape architecture, may store thermal energy as  [[solar pond]]s, and treat [[wastewater]] as [[treatment pond]]s. Ponds may be fresh, [[saltwater]], or [[brackish]].

A river is a natural flowing [[watercourse]], usually [[freshwater]], flowing under the influence of gravity on [[ocean]], [[lake]], another river, or into the ground. Small rivers can be referred to using names such as [[stream]], creek, brook, rivulet, and [[rill]]. There are no official definitions for the generic term river as applied to [[Geographical feature|geographic features]],<ref name="Geonames" /> Rivers are part of the [[hydrological cycle]]; water generally collects in a river from [[Precipitation (meteorology)|precipitation]] in a [[drainage basin]] from [[surface runoff]] and other sources such as [[groundwater recharge]], [[spring (hydrosphere)|springs]], and the release of stored water in natural ice and snow. Potamology is the scientific study of rivers, while [[limnology]] is the study of inland waters in general.

===Subterranean water===
{{main|Aquifer}}
[[File:Aquifer en.svg|thumb|Typical aquifer cross-section]]
<!-- [[File:Lechuguilla Cave Pearlsian Gulf.jpg|thumb|[[Lechuguilla Cave]], [[New Mexico]], United States]] -->
[[File:JB CAVE.png|thumb|Diving in a flooded cave]]
An '''aquifer''' is an underground layer of [[water]]-bearing [[permeability (earth sciences)|permeable rock]], rock fractures or unconsolidated materials ([[gravel]], [[sand]], or [[silt]]). The study of water flow in aquifers and the characterization of aquifers is called [[hydrogeology]].  If an impermeable layer overlies the aquifer, pressure could cause it to become a confined aquifer.

Aquifers may be classified as porous or [[karst]], where a porous aquifer contains the water in the spaces between the grains of a loose sediment or rock (typically [[sand]] or [[sandstone]]), while a karst aquifer contains water mainly in relatively large voids in relatively impermeable rock, such as [[limestone]] or [[Dolomite (rock)|dolomite]].<ref>{{Cite web|title=aquifer {{!}} Types & Facts|url=https://www.britannica.com/science/aquifer|access-date=2021-06-28|website=Encyclopedia Britannica|language=en}}</ref>

Water filled [[caves]] can be classified as active and relict: active caves have water flowing through them; relict caves do not, though water may be retained in them. Types of active caves include inflow caves ("into which a stream sinks"), outflow caves ("from which a stream emerges"), and through caves ("traversed by a stream").<ref name="Silvestru 2008" />

===Artificial bodies of water===
{{main|Dam|Reservoir|Canal}}
A reservoir is, most commonly, an enlarged natural or artificial lake, pond or impoundment created using a [[dam]] or [[Lock (water transport)|lock]] to store water. Reservoirs can be created in a number of ways, including controlling a watercourse that drains an existing body of water, interrupting a watercourse to form an embayment within it, through excavation, or building [[retaining wall]]s or [[levee]]s. Canals are artificial waterways which may have dams and locks that create reservoirs of low speed current flow.

==Physical characteristics==
{{main|Water|Properties of water}}

Water is a [[transparency and translucency|transparent]], [[taste]]less, [[odorless]], and [[Color of water|nearly colorless]] [[chemical substance]]. Its [[chemical formula]] is H<sub>2</sub>O, meaning that each of its [[molecule]]s contains one [[oxygen]] and two [[hydrogen]] [[atom]]s, connected by [[covalent bond]]s. Water is the name of the liquid state of H<sub>2</sub>O at [[standard ambient temperature and pressure]]. Water at the surface of the Earth moves continually through the [[water cycle]] of [[evaporation]], [[transpiration]] ([[evapotranspiration]]), [[condensation]], [[precipitation (meteorology)|precipitation]], and [[Surface runoff|runoff]], usually reaching the sea. Water seldom exists in a pure form, it almost always contains dissolved substances, and usually other matter in suspension.

===Density===
{{see also|Properties of water#Water, ice and vapor}}
[[File:Density of ice and water (en).svg|thumb|Density of ice and water as a function of temperature]]
The [[density]] of water is about {{convert|1|g/cm3|lb/ft3}} The density varies with temperature, but not linearly: as the temperature increases, the density rises to a peak at  {{convert|3.98|°C|°F}} and then decreases; this is unusual.<ref name="Greenwood and Earnshaw 1997" /> Regular, [[Ice Ih|hexagonal ice]] is also less dense than liquid water—upon freezing, the density of water decreases by about 9%.<ref name="Perlman" /> These effects are due to the reduction of thermal motion with cooling, which allows water molecules to form more hydrogen bonds that prevent the molecules from coming close to each other.<ref name="Greenwood and Earnshaw 1997" /> While below 4&nbsp;°C the breakage of hydrogen bonds due to heating allows water molecules to pack closer despite the increase in the thermal motion (which tends to expand a liquid),  above 4&nbsp;°C water expands as the temperature increases.<ref name="Greenwood and Earnshaw 1997" /> Water near the boiling point is about 4% less dense than water at {{convert|4|°C|°F}}.<ref name="Perlman" />{{efn|(1-0.95865/1.00000) × 100% {{=}} 4.135%}}

[[File:Anomalous expansion of water Summer Winter.svg|thumb|Temperature distribution in a lake in summer and winter]]
The unusual density curve and lower density of ice than of water is vital to life—if water were most dense at the freezing point, then in winter the very cold water at the surface of lakes and other water bodies would sink, the lake could freeze from the bottom up, and all life in them would be killed.<ref name="Perlman" /> Furthermore, given that water is a good thermal insulator (due to its&nbsp;heat capacity), some frozen lakes might not completely thaw in summer.<ref name="Perlman" /> The layer of ice that floats on top insulates the water below.<ref name="Zumdahl and Zumdahl 2013" /> Water at about 4&nbsp;°C (39&nbsp;°F) also sinks to the bottom, thus keeping the temperature of the water at the bottom constant (see diagram).<ref name="Perlman" />

The density of sea water depends on the dissolved salt content as well as the temperature. Ice still floats in the oceans, otherwise they would freeze from the bottom up. The salt content lowers the freezing point by about 1.9&nbsp;°C and lowers the temperature of the density maximum of water to the fresh water freezing point at 0&nbsp;°C.<ref name="NOAA 2" /> This is why, in ocean water, the downward convection of colder water is ''not'' blocked by an expansion of water as it becomes colder near the freezing point. The oceans' cold water near the freezing point continues to sink. So creatures that live at the bottom of cold oceans like the [[Arctic Ocean]] generally live in water  4&nbsp;°C colder than at the bottom of frozen-over [[fresh water]] lakes and rivers.

As the [[surface science|surface]] of sea water begins to freeze (at −1.9&nbsp;°C for salinity 3.5%) the ice that forms is essentially salt-free, with about the same density as freshwater ice.<ref name="NOAA 2" /> This ice floats on the surface, and the salt that is "frozen out" adds to the [[salinity]] and density of the sea water just below it, in a process known as ''[[brine rejection]]''. This denser salt water sinks by convection. This produces essentially freshwater ice at −1.9&nbsp;°C on the surface.<ref name="NOAA 2" /> On a large scale, the process of brine rejection and sinking cold salty water results in ocean currents forming to transport such water away from the Poles, leading to a global system of currents called the [[thermohaline circulation]].

====Pressure====
{{see also|Hydrostatic pressure}}
The [[density]] of water causes ambient pressures that increase dramatically with depth. The [[atmospheric pressure]] at the surface is 14.7 pounds per square inch or around 100&nbsp;kPa. A comparable hydrostatic pressure occurs at a depth of only {{convert|10|m|ft}} ({{convert|9.8|m|ft}} for sea water). Thus, at about 10 m below the surface, the water exerts twice the pressure (2 atmospheres or 200&nbsp;kPa) as air at surface level.

====Buoyancy====
{{main|Buoyancy}}
Any object immersed in water is subjected to a [[buoyancy|buoyant]] force that counters the force of [[gravity]], appearing to make the object less heavy. If the overall density of the object exceeds the density of water, the object sinks. If the overall density is less than the density of water, the object rises until it floats on the surface.

===Penetration of light===
[[image:Nur05018-Pillow lavas off Hawaii.jpg|thumb|right|300px|Note the bluish cast given to objects in this underwater photo of [[pillow lava]] ([[NOAA]]).]]
{{see also|Underwater vision#Visibility}}
With increasing depth underwater, [[sunlight]] is absorbed, and the amount of visible [[light]] diminishes. Because absorption is greater for long [[wavelength]]s (red end of the [[visible spectrum]]) than for short wavelengths (blue end of the visible spectrum), the [[Optical spectrum|colour spectrum]] is rapidly altered with increasing depth. White objects at the surface appear bluish underwater, and red objects appear dark, even black. Although light penetration will be less if water is [[turbidity|turbid]], in the very clear water of the open ocean less than 25% of the surface light reaches a depth of 10&nbsp;m (33&nbsp;feet). At 100&nbsp;m (330&nbsp;ft) the light present from the sun is normally about 0.5% of that at the surface.{{citation needed|date=June 2016}}

The [[Photic zone|euphotic depth]] is the depth at which light intensity falls to 1% of the value at the surface. This depth is dependent upon water clarity, being only a few metres underwater in a turbid estuary, but may reach up to 200 metres in the open ocean. At the euphotic depth, plants (such as [[phytoplankton]]) have no net energy gain from photosynthesis and thus cannot grow.

===Temperature===
There are three layers of ocean temperature: the [[Mixed layer#Oceanic mixed layer|surface layer]], the [[Thermocline#Oceans|thermocline]], and the [[Deep sea|deep ocean]]. The average temperature of surface layer is about 17&nbsp;°C. About 90% of ocean's water is below the thermocline in the deep ocean, where most of the water is below 4&nbsp;°C.<ref>{{cite web |title=Temperature of Ocean Water |website=Windows to the Universe |publisher=National Earth Science Teachers Association (NESTA) |url=https://www.windows2universe.org/?page=/earth/Water/temp.html}}</ref>

There are temperature anomalies at active [[volcano|volcanic sites]] and [[hydrothermal vent]]s, where deep-water temperatures can significantly exceed 100&nbsp;°C.

===Thermal conductivity===
Water [[thermal conductivity|conducts]] heat around 25 times more efficiently than air. [[Hypothermia]], a potentially fatal condition, occurs when the human body's core temperature falls below 35&nbsp;°C. Insulating the body's warmth from water is the main purpose of [[diving suit]]s and [[exposure suit]]s when used in water temperatures below 25&nbsp;°C.

===Acoustic properties===
{{see also|Underwater acoustics}}
[[Sound]] is transmitted about 4.3 [[Speed of sound|times faster in water]] (1,484&nbsp;m/s in fresh water) than in air (343&nbsp;m/s). The human brain can determine the direction of sound in air by detecting small differences in the time it takes for sound waves in air to reach each of the two ears. For these reasons, divers find it difficult to determine the direction of sound underwater. Some animals have adapted to this difference and many use sound to navigate underwater.

==Ecosystems==
{{main|Aquatic ecosystem|Freshwater ecosystem|Marine ecosystem}}
An [[aquatic ecosystem]] is an [[ecosystem]] in a [[body of water]]. [[Biocoenosis|Communities]] of [[biota (ecology)|organisms]] that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are [[marine ecosystem]]s and [[freshwater ecosystem]]s.<ref name="Alexander 1999" />

Marine ecosystems are the largest of [[Earth]]'s [[aquatic ecosystem]]s and are distinguished by waters that have a high salt content. Marine waters cover more than 70% of the surface of the Earth and account for more than 97% of Earth's water supply<ref name="Oceanic Institute" /><ref name="National Geographic" /> and 90% of habitable space on Earth.<ref name="UNESCO" /> Marine ecosystems include nearshore systems, such as the [[salt marsh]]es, [[mudflat]]s, [[seagrass meadow]]s, [[mangrove]]s, rocky [[Intertidal zone|intertidal systems]] and [[coral reefs]]. They also extend from the coast to include offshore systems, such as the surface [[ocean]], [[Pelagic zone|pelagic ocean]] waters, the [[deep sea]], [[Hydrothermal vent|oceanic hydrothermal vents]], and the [[Benthos|sea floor]]. Marine ecosystems are characterized by the biological [[Marine life|community of organisms]] that they are associated with and [[Marine habitats|their physical environment]]. As the world ocean is the principal component of Earth's hydrosphere, it is integral to [[life]], forms part of the [[carbon cycle]], and influences [[climate]] and [[weather]] patterns. The World Ocean is the [[habitat]] of 230,000 known [[species]], but because much of it is unexplored, the number of species that exist in the ocean is much larger, possibly over two million.<ref name="Drogin 2009" />
 
Freshwater ecosystems include [[Lake ecosystem|lakes]] and [[pond]]s, [[River ecosystem|rivers]], [[stream]]s, [[spring (hydrology)|spring]]s, [[aquifer]]s, [[bog]]s, and [[wetland]]s. They have a lower [[salt]] content than marine ecosystems. Freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation. Freshwater ecosystems can be divided into [[lentic ecosystem]]s (still water) and [[lotic ecosystem]]s (flowing water).<ref name="Wetzel 2001" />

Aquatic ecosystems are characterised by the limitation on ambient lighting due to absorption by the water itself and by dissolved and suspended matter in the water column, and by the support provided by buoyancy. Nutrients usable by plants are dissolved in the water, making them easily available. However, the interaction of light absorption by water, matter and living organisms themselves leads to very different light and light spectrum conditions depending on the respective ecosystem and its water depth. This affects photosynthesis and the ecology of plants and [[phytoplankton]].<ref>{{Cite journal |last1=Hintz |first1=Nils Hendrik |last2=Schulze |first2=Brian |last3=Wacker |first3=Alexander |last4=Striebel |first4=Maren |date=March 2022 |title=Ecological impacts of photosynthetic light harvesting in changing aquatic environments: A systematic literature map |journal=Ecology and Evolution |language=en |volume=12 |issue=3 |pages=e8753 |doi=10.1002/ece3.8753 |issn=2045-7758 |pmc=8939368 |pmid=35356568|bibcode=2022EcoEv..12E8753H }}</ref> Outside the euphotic zone, photosynthesis cannot occur and life must use other sources of energy than sunlight.

==Humans==
{{main|Underwater diving|Science of underwater diving}}
Although a number of [[human]] activities are conducted underwater&mdash;such as research, [[underwater diving]] for [[professional diving|work]] or [[recreational diving|recreation]], and [[underwater warfare]] with [[submarines]], the underwater environment is hostile to humans in many ways and therefore little explored.

An immediate obstacle to human activity under water is that human [[lung]]s cannot naturally function in this environment. Unlike the [[gill]]s of [[fish]], human lungs are adapted to the exchange of [[gas]]es at atmospheric [[pressure]]. Any penetration into the underwater environment for more than a few minutes requires [[Underwater breathing apparatus|artificial aids]] to maintain life.

For solid and liquid tissues like bone, muscle and blood, the high ambient pressure is not much of a problem; but it is a problem for any gas-filled spaces like the [[mouth]], [[ear]]s, [[paranasal sinus]]es and lungs. This is because the gas in those spaces is much more compressible than the solids and liquids, and reduces in volume much more when under pressure and so does not provide those spaces with support against the higher outside pressure. Even at a depth of {{convert|8|ft|m|abbr=on}} underwater, an inability to [[Ear clearing|equalize air pressure]] in the [[middle ear]] with outside water pressure can cause pain, and the [[tympanic membrane]] (eardrum) can rupture at depths under 10&nbsp;ft (3&nbsp;m). The danger of [[Barotrauma|pressure damage]] is greatest in shallow water because the ratio of pressure change is greatest near the surface of the water. The raised pressure also affects the solution of [[breathing gas]]es in the tissues over time, and can lead to a range of adverse effects, such as [[inert gas narcosis]], and [[oxygen toxicity]]. [[Decompression (diving)|Decompression]] must be controlled to avoid bubble formation in the tissues and the consequent symptoms of [[decompression sickness]].

With a few exceptions, the underwater environment tends to cool the unprotected human body. This heat loss will generally lead to hypothermia eventually.

===Hazards===
{{see also|Hazard|List of diving hazards and precautions|Human factors in diving safety|List of diving environments by hazard}}
There are several classes of hazards to humans inherent to the underwater environment. 
*Absence of breathable gas, which can cause [[asphyxia]], specifically by [[drowning]].
*Ambient pressures which could cause [[barotrauma]], or toxic effects of breathing gas components at raised partial pressures.
*Ambient temperatures which may lead to [[hypothermia]], or in unusual cases, to [[hyperthermia]], due to high rates of heat exchange.
*Solution of inert breathing gas components may lead to [[decompression sickness]] if [[Decompression (diving)|decompression]] is too rapid.
*Entrainment of diver by moving water in [[Ocean current|currents]] and [[Wind wave|waves]] can cause injury by impacting the diver against hard objects or moving them to inappropriate depths.
*[[List of diving hazards and precautions#The specific diving environment|Dangerous aquatic organisms]] of various sorts.

===Ambient pressure diving===
{{See also|Underwater diving|Underwater habitat}}
<!-- [[File:DIR Divers Sandra edwards 2010.JPG|thumb|Recreational divers are exposed to the ambient pressure]] -->
[[File:Tektite I exterior.jpg|thumb|Tektite I underwater habitat with ambient pressure divers using scuba]]
In ambient pressure diving, the diver is directly exposed to the pressure of the surrounding water. The ambient pressure diver may dive on breath-hold, or use breathing apparatus for [[scuba diving]] or [[surface-supplied diving]], and the [[saturation diving]] technique reduces the risk of [[decompression sickness]] (DCS) after long-duration deep dives. Immersion in water and exposure to cold water and high pressure have physiological effects on the diver which limit the depths and duration possible in ambient pressure diving. Breath-hold endurance is a severe limitation, and breathing at high ambient pressure adds further complications, both directly and indirectly. Technological solutions have been developed which can greatly extend depth and duration of human ambient pressure dives, and allow useful work to be done underwater.<ref name="ECHM-EDTM 2011" />

===Atmospheric pressure diving===
{{main|Atmospheric diving suit}}
[[File:Newtsuit atmospheric diving suit.jpg|thumb|right|The [[Newtsuit]] has fully articulated, rotary joints in the arms and legs. These provide great mobility, while remaining largely unaffected by high pressures.]]
A diver can be isolated from the ambient pressure by using an atmospheric diving suit (ADS), which is a small one-person articulated [[anthropomorphic]] [[submersible]] which resembles a [[plate armour|suit of armour]], with elaborate pressure resisting joints to allow articulation while maintaining an internal pressure of one atmosphere. An ADS can be used for relatively deep dives of up to {{convert|2300|ft|m}} for many hours, and eliminates the majority of significant physiological dangers associated with deep diving; the occupant need not decompress, there is no need for special gas mixtures, nor is there danger of [[decompression sickness]] or [[nitrogen narcosis]], and the diver is effectively isolated from most aquatic organisms.<ref>{{cite web |title=WASP Specifications |url=http://www.oceaneering.com/oceandocuments/brochures/subseaprojects/OPG%20-%20WASP.pdf |archive-url=https://web.archive.org/web/20140303201139/http://www.oceaneering.com/oceandocuments/brochures/subseaprojects/OPG%20-%20WASP.pdf |url-status=dead |archive-date=3 March 2014 |access-date=27 February 2014 }}</ref> Divers do not even need to be skilled swimmers, but mobility and dexterity are significantly degraded.

===Submersibles and submarines===
{{main|Submersible|Submarine}}
A submersible is a small [[watercraft]] designed to operate underwater. The term ''submersible'' is often used to differentiate from other underwater vessels known as [[submarine]]s, in that a submarine is a fully autonomous craft, capable of renewing its own power and breathing air, whereas a submersible is usually supported by a surface vessel, platform, shore team or sometimes a larger submarine. There are many types of submersibles, including both manned and unmanned craft, otherwise known as [[remotely operated vehicle]]s or ROVs.<ref name="Canadian Encyclopedia" />

=== Remotely operated or autonomous vehicles ===
{{see also|Remotely operated underwater vehicle|Autonomous underwater vehicle}}
[[File:ROV working on a subsea structure.jpg|thumb|ROV working on a subsea structure]]
[[File:MiniU.jpg|thumb|[[Pluto Plus]] AUV for underwater mine identification and destruction. From Norwegian minehunter KNM Hinnøy]]
Remotely operated underwater vehicles and autonomous underwater vehicles are part of a larger group of undersea systems known as [[unmanned underwater vehicle]]s. ROVs are unoccupied, usually highly maneuverable, and operated by a crew either aboard a vessel/floating platform or on proximate land. They are linked to a host ship by a neutrally buoyant [[tether]], or a load-carrying [[umbilical cable]] is used along with a tether management system (TMS). The umbilical cable contains a group of [[electrical conductor]]s and fiber optics that carry electric power, video, and data signals between the operator and the TMS. Where used, the TMS then relays the signals and power for the ROV down the tether cable. Once at the ROV, the electric power is distributed between the components of the ROV. In high-power applications, most of the electric power drives a high-power electric motor which drives a [[hydraulic pump]] for propulsion and to power equipment. Most ROVs are equipped with at least a video camera and lights. Additional equipment is commonly added to expand the vehicle's capabilities. Autonomous underwater vehicles (AUVs) are [[robot]]s that travel underwater without requiring input from an operator. [[Underwater glider]]s are a subclass of AUVs.<ref name="ROV" />

==Sciences==
*{{annotated link|Hydrology}}
*{{annotated link|Hydrography}}
*{{annotated link|Limnology}}
*{{annotated link|Marine biology}}
*{{annotated link|Marine chemistry}}
*{{annotated link|Marine ecology}}
*{{annotated link|Marine geology}}
*{{annotated link|Oceanography}}
*{{annotated link|Potamology}}
*{{annotated link|Underwater archaeology}}

==See also==
*{{annotated link|Science of underwater diving}}
*{{annotated link|Subsurface ocean current}}
*{{annotated link|Timeline of diving technology}}
*{{annotated link|Underwater acoustics}}
*{{annotated link|Underwater photography}}
*{{annotated link|Underwater vision}}
*{{annotated link|UNESCO Convention on the Protection of the Underwater Cultural Heritage}}

==References==
{{reflist|refs=

<ref name="Alexander 1999" >{{cite book| last = Alexander| first = David E.| title = Encyclopedia of Environmental Science| publisher = [[Springer Science+Business Media|Springer]]| date = 1 May 1999| isbn = 0-412-74050-8 }}</ref>

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}}

==Footnotes==
{{Notelist}}

{{Underwater diving|scidiv}}
{{authority control}}

{{Commons category|Underwater}}

[[Category:Environments]]
[[Category:Oceanography]]
[[Category:Underwater diving environment]]