An underwater scene just beneath the surface.

Underwater is a term describing the realm below the surface of water where the water exists in a natural feature (called a body of water) such as an ocean, sea, lake, pond, or river. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Water is a common Chemical substance that is essential for the survival of all known forms of Life. An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. This article is about the body of water For other uses see SEA and Seas. A lake (from Latin lacus) is a Terrain feature (or Physical feature) a body of Liquid on the surface of a world that is localized to the A pond is a body of water smaller than a Lake, both being examples of Terrain features Although the term pond is universally used to describe waterbodies that "Riverine" redirects here For the use of that term in Maritime geography, see there Three quarters of the planet Earth is covered by water. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 A majority of the planet's solid surface is abyssal plain, at depths between 4000 and 5500 m (13,000 to 18,000 ft) below the surface of the oceans. Abyssal plains are flat or very gently sloping areas of the deep Ocean basin floor The solid surface location on the planet closest to the center of the orb is the Challenger Deep, located in the Mariana Trench at a depth of 10,924 m (35,838 ft) under the sea. The Challenger Deep is the deepest surveyed point in the oceans with a depth of about 11000 metres (about 36000 feet The Mariana Trench (or Mariana's Trench) is the deepest part of the world's Oceans and the deepest location on the surface of the Earth 's To help compare different Orders of magnitude this page lists Lengths between 10 and 100 km (104 to 105 m)

Although a number of human activities are conducted underwater—such as scuba diving for work or recreation, or even underwater warfare with submarines—this very extensive environment on planet earth is hostile to humans in many ways and therefore little explored. Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set. Hardhat diving redirects here Often "hardhat diving" is used to specifically mean diving in the old-type Standard diving dress. Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment Underwater warfare refers to Combat conducted Underwater such as Actions by Submarines actions and Anti-submarine warfare, i A submarine is a Watercraft that can operate independently below water as distinct from a Submersible that has only limited underwater capability EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001

An immediate obstacle to human activity under water is the fact that human lungs cannot naturally function in this environment. lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive Unlike the gills of fish, human lungs are adapted to the exchange of gases at atmospheric pressure, not liquids. A gill is an anatomical structure found in many aquatic organisms Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive This page is about the physical properties of gas as a state of matter Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of Aside from simply having insufficient musculature to rapidly move water in and out of the lungs, a more significant problem for all air breathing animals, such as mammals and birds, is that water contains so little dissolved oxygen compared with atmospheric air. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Air is around 21% O₂; water typically is less than 0. 001% dissolved oxygen.

The density of water also causes problems that increase dramatically with depth. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different The air pressure at the planet surface is 101 kilopascals (14. 7 lb. per sq. inch). A comparable water pressure occurs at a depth of only 10 m (34 ft. ) (9. 8 m (33 ft) for sea water) Thus, at about 10 m below the surface the water exerts twice the pressure (2 atmospheres or 200 kPa) on the body as air at surface level.

For solid objects like our bones and muscles, this added pressure is not much of a problem; but it is a problem for any air-filled spaces like the mouth, ears, paranasal sinuses and lungs. The mouth, buccal cavity, or oral cavity is the first portion of the Alimentary canal that receives food and begins digestion by mechanically breaking up The ear is the sense organ that detects Sounds The Vertebrate ear shows a common biology from Fish to Humans with variations Paranasal sinuses are air-filled spaces communicating with the nasal cavity within the bones of the Skull and face This is because the air in those spaces reduces in volume when under pressure and so does not provide those spaces with support from the higher outside pressure. Even at a depth of 8 feet (2. 5 m) underwater, an inability to equalize air pressure in the middle ear with outside water pressure can cause pain, and the tympanic membrane can rupture at depths under 10 ft (3 m). The middle ear is the portion of the ear internal to the Eardrum, and external to the Oval window of the Cochlea. The tympanic membrane (also tympanum or myrinx is a thin membrane that separates the External ear from the Middle ear. The danger of pressure damage is greatest in shallow water because the rate of pressure change is greatest at the surface of the water. For example the pressure increase between the surface and 10 m (33 ft) is 100% (100 kPa to 200 kPa), but the pressure increase from 30 m (100 ft) to 40 m (130 ft) is only 25% (400 kPa to 500 kPa).

Any object immersed in water is provided with a buoyant force that counters the force of gravity, appearing to make the object less heavy. In Physics, buoyancy ( BrE IPA: /ˈbɔɪənsi/ is the upward Force on an object produced by the surrounding liquid or gas in which it is Gravitation is a natural Phenomenon by which objects with Mass attract one another 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.

Note the bluish cast given to objects in this underwater photo of pillow lava. Lava is molten rock expelled by a Volcano during an eruption When first expelled from a volcanic vent it is a Liquid at Temperatures (NOAA)

With increasing depth underwater, sunlight is absorbed, and the amount of visible light diminishes. The National Oceanic and Atmospheric Administration ( NOAA) is a scientific agency within the United States Department of Commerce focused on the conditions of the Sunlight, in the broad sense is the total spectrum of the Electromagnetic radiation given off by the Sun. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Because absorption is greater for long wavelengths (red end of the visible spectrum) than for short wavelengths (blue end of the visible spectrum), the colour spectrum is rapidly altered with increasing depth. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. White objects at the surface appear bluish underwater, and red objects appear dark, even black. Although light penetration will be less if water is turbid, in the very clear water of the open ocean less than 25% of the surface light reaches a depth of 10 m (33 feet). Turbidity is the cloudiness or haziness of a fluid caused by individual particles ( suspended solids) that are generally invisible to the Naked eye At 100 m (330 ft) the light present from the sun is about 0. 5% of that at the surface.

The 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 meters underwater in a turbid estuary, but may reach 200 meters in the open ocean. At the euphotic depth, plants (such as phytoplankton) have no net energy gain from photosynthesis and thus cannot grow. Phytoplankton are the Autotrophic component of the Plankton community

At depths greater than a few hundred meters, the sun has little effect on water temperature, because the sun's energy has been absorbed by water at the surface. In the great depths of the ocean the water temperature is very cold. In fact, 75% of the water in the world ocean (the great depths) has a temperature between 0 °C and 2 °C.

Water conducts heat around twenty five times more efficiently than air. In Physics, thermal conductivity, k is the property of a material that indicates its ability to conduct Heat. Hypothermia, a potentially fatal condition, occurs when the human body's core temperature falls below 35 °C. Hypothermia is a condition in which an organism's temperature drops below that required for normal Metabolism and bodily functions 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. A diving suit is a garment or device designed to protect a diver from the Underwater environment

Sound is transmitted about 4. Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies 5 times faster in water (about 1435 m/s in fresh water) as it is in air (330 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.

See also

• Timeline of underwater technology
• Underwater photography
• Underwater acoustics

References

• Dueker, C. This is a Timeline of Underwater Technology. The entries marked ## are about Decompression tables Pre-industrial Underwater photography is the process of taking Photographs while under water Underwater acoustics is the study of the propagation of Sound in Water and the interaction of the mechanical waves that constitute sound with the water and its boundaries W. 1970, Medical aspects of sport diving. A. S. Barnes and Co. , New York. 232 pp.
• The Briny Deep – Oceanography notes at San Jose State University.