Underwater environment

thumb|250px|[[Great Barrier Reef, Australia]]

An underwater environment is an environment of, and immersed in, liquid water in a natural or artificial feature (called a 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 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

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 below the surface of the oceans. The solid surface location on the planet closest to the centre of the geoid is the Challenger Deep, located in the Mariana Trench at a depth of . 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.

Oceans and seas

thumb|Clouds over the Atlantic Ocean

An ocean is a body of water that composes much of a planet's hydrosphere. On Earth, an ocean is one of the major conventional divisions of the World Ocean. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans. 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, though "the sea" refers also to the oceans.

Saline water covers approximately 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. The ocean contains 97% of Earth's water, and oceanographers have stated that less than 100% of the World Ocean has been explored.

Lakes, ponds, and rivers

thumb|[[Samur River in Azerbaijan – In the natural landscape]]

A lake is an area filled with water, localized in a basin, that is surrounded by land, apart from any river or other outlet that serves to feed or drain the lake.

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.

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").

There are temperature anomalies at active volcanic sites and hydrothermal vents, where deep-water temperatures can significantly exceed 100 °C.

Thermal conductivity

Water 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 °C. Insulating the body's warmth from water is the main purpose of diving suits and exposure suits when used in water temperatures below 25 °C.

Acoustic properties

Sound is transmitted about 4.3 times faster in water (1,484 m/s in fresh water) than in air (343 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

An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems. Outside the euphotic zone, photosynthesis cannot occur and life must use other sources of energy than sunlight.

Humans

Although a number of human activities are conducted underwater—such as research, underwater diving for work or 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 lungs cannot naturally function in this environment. Unlike the gills of fish, human lungs are adapted to the exchange of gases at atmospheric pressure. Any penetration into the underwater environment for more than a few minutes requires 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, ears, paranasal sinuses 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 underwater, an inability to 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 ft (3 m). The danger of 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 gases in the tissues over time, and can lead to a range of adverse effects, such as inert gas narcosis, and oxygen toxicity. 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

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 is too rapid.
Entrainment of diver by moving water in currents and waves can cause injury by impacting the diver against hard objects or moving them to inappropriate depths.
Dangerous aquatic organisms of various sorts.

Ambient pressure diving

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. Divers do not even need to be skilled swimmers, but mobility and dexterity are significantly degraded.

Submersibles and submarines

A submersible is a small watercraft designed to operate underwater. The term submersible is often used to differentiate from other underwater vessels known as submarines, 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 vehicles or ROVs.

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