Underwater environment
The underwater environment is the region below the surface 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. However, the underwater environment is hostile to humans in many ways and often inaccessible, and therefore relatively little explored.
Three quarters of the planet Earth is covered by water. Most 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 center of the geoid is the Challenger Deep, located in the Mariana Trench at a depth of 10,924 metres (35,840 ft). 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.[citation needed]
An ocean is a body of water that composes much of a planet's hydrosphere.[1] On Earth, an ocean is one of the major conventional divisions of the World Ocean. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans.[2][3] The word "ocean" is often used interchangeably with "sea" in American English. Strictly speaking, a sea is a body of water (generally a division of the world ocean) partly or fully enclosed by land,[4] though "the sea" refers also to the oceans.
Saline water covers approximately 361,000,000 km2 (139,000,000 sq mi) and is customarily divided into several principal oceans and smaller seas, with the ocean covering approximately 71% of Earth's surface and 90% of the Earth's biosphere.[5] The ocean contains 97% of Earth's water, and oceanographers have stated that less than 5% of the World Ocean has been explored.[5] The total volume is approximately 1.35 billion cubic kilometers (320 million cu mi) with an average depth of nearly 3,700 meters (12,100 ft).[6][7][8]
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