Underwater, sometimes shortened as U/W, 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. Three quarters of the planet earth is covered by water. 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. 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.

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.

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.

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♫ Amongspoons - Underwater Moon
spacestationsma (Space Station Soma) Tue, 08 Dec 2009 23:18:20 -0000
♫ Amongspoons - Underwater Moon
I just got out the shower...more like underwater meditation. I got alot off my mind in there.
Tai2flyyy (♫♪ Tai K. ♫♪) Tue, 08 Dec 2009 23:17:50 -0000
I just got out the shower...more like underwater meditation. I got alot off my mind in there.
RT @DerekMarkham: RT @greengirl19 First-ever unmanned underwater glider crosses the Atlantic http://ow.ly/K0eJ
naturapprentice (Carmel Marshall) Tue, 08 Dec 2009 23:17:26 -0000
RT @DerekMarkham: RT @greengirl19 First-ever unmanned underwater glider crosses the Atlantic http://ow.ly/K0eJ
http://twitpic.com/spz8g - Underwater Gator Cake @ the Univ store
GroovyDeals (Ben & Jerry's) Tue, 08 Dec 2009 23:16:13 -0000
http://twitpic.com/spz8g - Underwater Gator Cake @ the Univ store
local news: Underwater Data Back Up Theory of Hawaii’s Origin http://bit.ly/8CcuTj
viil (Viil Lid) Tue, 08 Dec 2009 23:14:22 -0000
local news: Underwater Data Back Up Theory of Hawaii’s Origin http://bit.ly/8CcuTj
Swim class wit the kids... Gotti is gettin ova her underwater fear...
Philly_Da_Boss (Phillip Bryant) Tue, 08 Dec 2009 23:14:14 -0000
Swim class wit the kids... Gotti is gettin ova her underwater fear...

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