Underwater

Helping orphans the way you would do it

Underwater, sometimes shortened as U/W, is a term describing the area below the surface of water, usually in a natural feature such as an ocean, sea, lake, pond, or river. Although a number of 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.

The fact that human lungs cannot properly function underwater is but one reason we cannot easily visit the underwater realm. Aside from our simply having insufficient musculature to rapidly move water in and out of the lungs, a more significant problem for all terrestrial animals is that water contains so little dissolved oxygen compared with atmospheric air. Air is around 20% O₂; water typically is less than 0.001% dissolved oxygen. The weight of water also causes problems that increase dramatically with depth. The weight of a column of air 1 square inch in cross-section and extending from the planet surface all the way to the top of the atmosphere is around 14.7 lbs. A comparable weight (and thus pressure) of water is only 10 m or 34 ft (33 for sea water) high. Thus, the water at 33 or 34 feet deep underwater exerts twice the pressure (2 atmospheric pressure) on the body as air at the water surface (1 atmospheric pressure). 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 and lungs. 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 (although rarely) at depths under 10 ft (3 m). The greatest pressure differential per foot develops over the first 33 feet (Dueker 1970).

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 color 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 about 0.5% of that at the surface.

Because all of the sun's energy is absorbed by the surface water, below a few hundred meters the sun has little effect on water temperature. 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 and 2°C.

Three quarters of the planet earth is covered by water. A majority of the ocean bottom is the abyssal plain, at depths between 4000 and 5500 m (13,000 to 18,000 ft). The deepest surface location on the planet is underwater: it is the Challenger Deep located in the Mariana Trench at a depth of 10,924 m (35,838 ft) under the surface.

See also: Timeline of underwater technology

References

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.

The term underwater might also refer to:

Underwater – a popular trance anthem by Delerium.
Underwater (band) – a band formed in the late 1990s with Jeremy Wilkins and Melissa Mileski.
Underwater – a graphic novel by Chester Brown.