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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/07/15 17:33:10」(JST)

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Underwater is 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. Three quarters of the planet Earth is covered by water. A majority of the planet's solid surface is abyssal plain, at depths between 4,000 and 5,500 metres (13,100 and 18,000 ft) below the surface of the oceans. The solid surface location on the planet closest to the centre of the orb is the Challenger Deep, located in the Mariana Trench at a depth of 10,924 metres (35,840 ft). Although a number of human activities are conducted underwater—such as research, 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. But it can be explored by sonar, or more directly via manned 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, in the name of detecting enemy submarines, or aiding friendly ones, though the resulting maps may still be classified.

An immediate obstacle to human activity under water is the fact 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, not liquids. 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. 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 atmospheric pressure at the surface is 14.7 pounds per square inch or around 100 kPa. A comparable water pressure occurs at a depth of only 10 m (33 ft) (9.8 m (32 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 human 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. 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 ft (2.4 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 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. 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 (NOAA).

With increasing depth underwater, sunlight is absorbed, and the amount of visible light diminishes. 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. 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). At 100 m (330 ft) the light present from the sun is normally 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 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.

At depths greater than a few hundred metres, 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 low. 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. 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.

Sound is transmitted about 4.3 times faster in water (1,484 m/s in fresh water) as it is 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. However, some animals have adapted to this difference and many use sound to navigate underwater.

Underwater life of Klein Bonaire - composite of 8 photos from snorkeling on a flat coral reef off the islet Klein Bonaire

References

Wikimedia Commons has media related to Underwater.

Dueker, C. 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. sjsu.edu

UpToDate Contents

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1. 高圧酸素療法 hyperbaric oxygen therapy
2. 小児における耳痛の評価 evaluation of earache in children
3. スキューバダイビングの合併症 complications of scuba diving
4. 一酸化炭素中毒 carbon monoxide poisoning

English Journal

Using accelerometers to determine the calling behavior of tagged baleen whales.

Goldbogen JA1, Stimpert AK2, DeRuiter SL3, Calambokidis J4, Friedlaender AS5, Schorr GS4, Moretti DJ6, Tyack PL7, Southall BL8.
The Journal of experimental biology.J Exp Biol.2014 Jul 15;217(Pt 14):2449-55. doi: 10.1242/jeb.103259. Epub 2014 May 6.
Low-frequency acoustic signals generated by baleen whales can propagate over vast distances, making the assignment of calls to specific individuals problematic. Here, we report the novel use of acoustic recording tags equipped with high-resolution accelerometers to detect vibrations from the surface
PMID 24803468

Neurotrophic and neuroprotective potential of human limbus-derived mesenchymal stromal cells.

Liang CM1, Weng SJ2, Tsai TH3, Li IH4, Lu PH5, Ma KH2, Tai MC6, Chen JT6, Cheng CY7, Huang YS8.
Cytotherapy.Cytotherapy.2014 Jul 2. pii: S1465-3249(14)00633-1. doi: 10.1016/j.jcyt.2014.05.015. [Epub ahead of print]
BACKGROUND AIMS: The purpose of this study was to examine neurotrophic and neuroprotective effects of limbus stroma-derived mesenchymal stromal cells (L-MSCs) on cortical neurons in vitro and in vivo.METHODS: Cultured L-MSCs were characterized by flow cytometry and immunofluorescence through the use
PMID 24996595

Terrestrial stress analogs for spaceflight associated immune system dysregulation.

Crucian B1, Simpson RJ2, Mehta S3, Stowe R4, Chouker A5, Hwang SA6, Actor JK6, Salam AP7, Pierson D8, Sams C9.
Brain, behavior, and immunity.Brain Behav Immun.2014 Jul;39C:23-32. doi: 10.1016/j.bbi.2014.01.011. Epub 2014 Jan 24.
Recent data indicates that dysregulation of the immune system occurs and persists during spaceflight. Impairment of immunity, especially in conjunction with elevated radiation exposure and limited clinical care, may increase certain health risks during exploration-class deep space missions (i.e. to
PMID 24462949

Japanese Journal

デジタル地球儀を用いた成長型海洋生態系アーカイブのデザイン手法

高田百合奈,朴婉寧,蜂谷聖未 [他],高田健太郎,西田志帆,渡邉英徳,植田佳樹
日本バーチャルリアリティ学会論文誌 17(3), 253-260, 2012-09-30
… In order to achieve the purpose of this study, it is valid to be displayed overlaid on the digital globe with data submitted by users, three-dimensional terrain models of the seabed, and photographs undersea. …
NAID 110009517480

デジタル地球儀を用いた成長型海洋生態系アーカイブのデザイン手法

高田百合奈,渡邉英徳,植田佳樹
情報処理学会研究報告. 人文科学とコンピュータ研究会報告 2012-CH-95(6), 1-6, 2012-07-28
本研究では,海洋生態系に関する情報のデジタルアーカイブの手法の提案を行う.海洋生態系に関する情報は,時間軸も含む多元的情報である.しかし,これらの情報を俯瞰的に閲覧できるようにデジタルアーカイブを行う適切な手段は存在しなかった.そこで,適切なVR技術を用いることで,海洋に関する多元的な情報の相互参照と,ユーザによるデジタルアーカイブの構築への参加を可能とする,成長型コンテンツを確立することを目的と …
NAID 110009424959

鉄道のトンネル(海面下のトンネル) (特集地下(地中)施設の電気設備)

高井渉
電気設備学会誌 30(7), 470-473, 2012-07-00
NAID 40019387375

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