Underwater is a term telling what is below the surface of a body of water such as an ocean, sea, lake, pond, or river. Most of the planet Earth is covered by water. The bottom of the ocean is called the sea-bed. Most of the sea-bed is between 4,000 and 5,500 metres (13,000 and 18,000 ft) below the surface of the ocean. Just like the Earth's dry land, the sea-bed also has mountains, valleys and flat plains, although they are covered with water. The deepest part of all the world's oceans is a place called the Challenger Deep, which is in the Mariana Trench. It is 10,924 metres (35,840 ft) under the sea.
Although a number of human activities happen underwater, such as scuba diving for work or recreation, or even warfare with submarines, most of the oceans, especially the deepest parts, have not been explored yet.