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 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 under the sea.