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. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern, and Arctic Oceans.Around 70% of the Earth's surface is covered by oceans. The world's oceans contain enough water to fill a cube with edges over 1000 kilometres (621 miles) in length. Ocean tides are caused by the Earth rotating while the Moon and Sun's gravitational pull acts on ocean water.Over vast periods of time, our primitive oceans formed. Water remained a gas until the Earth cooled below 212 degrees Fahrenheit . At this time, about 3.8 billion years ago, the water condensed into rain which filled the basins that are now our oceans.