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.
Seas and oceans are the only unexplored environment by man. Although the sea and the oceans take up most of the planet earth human technology still can not overcome the effect of the ocean. Ocean means to show your teeth stronger and more powerful and the largest overseas vessels which proved countless times. In spite of all the ocean can be seen a lot of beautiful creatures and species of fish of different colors and shapes that can delight everyone.