When the Earth formed billions of years ago, much of the water arrived via comet impacts and volcanic outgassing. This water filled the vast low-lying areas that eventually became oceans.

Oceans are like giant basins covering about 71% of Earth's surface, allowing them to hold an immense volume of water. Earth's gravity holds the water on the surface, and tectonic forces have shaped Earth’s crust into oceanic and continental regions. The ocean basins are the lowest points on Earth’s surface, so gravity draws water toward them, where it accumulates and remains.

Water constantly cycles through the atmosphere, land, and oceans, but it largely returns to the oceans due to gravity and the geography of Earth’s surface. Rivers, lakes, and glaciers represent only about 3% of Earth’s water, with freshwater lakes and rivers accounting for less than 1%.

Ocean water is salty because it has accumulated dissolved minerals, especially sodium chloride (table salt), over millions of years. This saline water makes up the bulk of Earth's water reserves, but it is difficult and energy-intensive to convert into fresh water for human use. salts are deposited on the sea floor as sediments and can be used by various species to create shells; this is partly why the ocean salinity is relatively consistent and does not keep increasing