Underwater World By Alexander Safonov

Underwater 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 4,000 and 5,500 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 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.