Underwater environment
An underwater environment is a environment of, and immersed in, liquid water in a natural or artificial feature (called a body of water), such as an ocean, sea, lake, pond, reservoir, river, canal, or aquifer. Some characteristics of the underwater environment are universal, but many depend on the local situation. Liquid water has been present on Earth for most of the history of the planet. The underwater environment is thought to be the place of the origin of life on Earth, and it remains the ecological region most critical to the support of life and the natural habitat of the majority of living organisms. Several branches of science are dedicated to the study of this environment or specific parts or aspects of it. A number of human activities are conducted in the more accessible parts of the underwater environment. These include research, underwater diving for work or recreation, and underwater warfare with submarines. It is hostile to humans in many ways and often inaccessible, and therefore relatively little explored. Three quarters of the planet Earth are covered by water. Most of the planet's solid surface is abyssal plain, at depths between below the surface of the oceans. The solid surface location on the planet closest to the center of the geoid is the Challenger Deep, located in the Mariana Trench at a depth of . There is a smaller part of the surface covered by bodies of fresh water and a large volume of underground water in aquifers. The underwater environment is hostile to humans in many ways and therefore little explored. It can be mapped by sonar, or more directly explored via manned, remotely operated, 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, to assist in navigating and detecting submarines, though the resulting maps may be classified. Ocean and Sea An ocean is a body of water that composes much of a planet's hydrosphere. On Earth, an ocean is one of the major conventional divisions of the World Ocean.