Undersea
Underwater refers to the region 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.
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