Underwater is a term describing the realm below the surface of water where the water exists in a natural feature 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 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 under the sea. 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. 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.