Underwater environment

Several branches of science are dedicated to the study of this environment or specific parts or aspects of it.

It can be mapped by sonar, or more directly explored via manned, remotely operated, or autonomous submersibles.

These are, in descending order by area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans.

[5] The total volume is approximately 1.35 billion cubic kilometers (320 million cu mi) with an average depth of nearly 3,700 meters (12,100 ft).

[9] Lakes lie on land and are not part of the ocean, and therefore are distinct from lagoons, and are also larger and deeper than ponds, though there are no official or scientific definitions.

In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the 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 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.

[11] It may arise naturally in floodplains as part of a river system, or be a somewhat isolated depression (such as a kettle, vernal pool, or prairie pothole).

Small rivers can be referred to using names such as stream, creek, brook, rivulet, and rill.

[15] A reservoir is, most commonly, an enlarged natural or artificial lake, pond or impoundment created using a dam or lock to store water.

Canals are artificial waterways which may have dams and locks that create reservoirs of low speed current flow.

Its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds.

Water seldom exists in a pure form, it almost always contains dissolved substances, and usually other matter in suspension.

[16] Regular, hexagonal ice is also less dense than liquid water—upon freezing, the density of water decreases by about 9%.

[17] Furthermore, given that water is a good thermal insulator (due to its heat capacity), some frozen lakes might not completely thaw in summer.

[19] 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.

[19] 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.

With increasing depth underwater, sunlight is absorbed, and the amount of visible light diminishes.

Hypothermia, a potentially fatal condition, occurs when the human body's core temperature falls below 35 °C.

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.

[25] Freshwater ecosystems include lakes and ponds, rivers, streams, springs, aquifers, bogs, and wetlands.

Freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation.

Unlike the gills of fish, human lungs are adapted to the exchange of gases at atmospheric pressure.

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.

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.

An ADS can be used for relatively deep dives of up to 2,300 feet (700 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.

There are many types of submersibles, including both manned and unmanned craft, otherwise known as remotely operated vehicles or ROVs.

ROVs are unoccupied, usually highly maneuverable, and operated by a crew either aboard a vessel/floating platform or on proximate land.