The water that makes up Earth's oceans is thought to have originated from meteors, comets, and volcanoes. Today, there are 5 major world oceans (the Pacific, Atlantic, Indian, Arctic, and Southern).

Salinity is the measure of how salty the oceans are. Ocean salinity is not constant around the globe. For example, salinity is higher in the subtropical oceans because the rate of evaporation is higher than the rate of precipitation. In the tropics, the opposite is true. Dissolved minerals that make up the salts in the world's oceans come primarily from the weathering and erosion of rocks on land and from volcanic eruptions.

Other physical properties of the oceans include waves, tides, and currents. Waves and surface currents are influenced by the wind. Deep ocean currents, known as thermohaline currents, are influenced by water density and temperature. The combination of ocean and atmospheric currents can influence weather & climate patterns. El Niño is a climate phenomenon that occurs in the Pacific Ocean, and is a result of the interaction between deep and surface ocean currents, as well as wind patterns and atmospheric conditions.

The next page describes waves, tides, currents, coastal processes & landforms, and changes in sea level in more detail.