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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RT @bryansyn: The Underwater Cellphone System http://bit.ly/1I3wlg
jomenat (jo menat) Thu, 05 Nov 2009 09:04:27 -0000
RT @bryansyn: The Underwater Cellphone System http://bit.ly/1I3wlg
omggggggg looolll this is amazing. i love the underwater world shit. weeeee.
glamchemy (Isa) Thu, 05 Nov 2009 09:04:11 -0000
omggggggg looolll this is amazing. i love the underwater world shit. weeeee.
The Underwater Cellphone System http://bit.ly/1I3wlg
bryansyn (Bryan S) Thu, 05 Nov 2009 09:01:33 -0000
The Underwater Cellphone System http://bit.ly/1I3wlg
Pictures from @iujaz and @millzero "Maldives Goes from Underwater Meetings to Huge Wind Farm" http://surl.me/teei
gaanagaa (Gaanagaa / Waddey) Thu, 05 Nov 2009 08:56:06 -0000
Pictures from @iujaz and @millzero "Maldives Goes from Underwater Meetings to Huge Wind Farm" http://surl.me/teei
Underwater Upkeep: Helping Biscayne National Park's Reefs http://bit.ly/4o1UAU
where_to_go (Travel Guide) Thu, 05 Nov 2009 08:42:35 -0000
Underwater Upkeep: Helping Biscayne National Park's Reefs http://bit.ly/4o1UAU
Underwater Upkeep: Helping Biscayne National Park's Reefs http://bit.ly/2QwZLB
travel4idiots (Travel Guide) Thu, 05 Nov 2009 08:42:33 -0000
Underwater Upkeep: Helping Biscayne National Park's Reefs http://bit.ly/2QwZLB

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