Ocean is the great body of water that covers nearly 71 percent of Earth's surface. People also call it the sea. The ocean contains 97 percent of all the water on Earth.

The ocean serves as a source of food, energy, minerals, and recreation. Ships sail the ocean to carry cargo between continents. The sea also plays a major role in Earth's climate system by regulating the air temperature and by supplying the moisture for rainfall. If there were no ocean, life could not exist on our planet.

The bottom of the ocean has features as varied as those on land. Huge plains spread across the ocean floor, and long mountain chains rise toward the surface. Volcanoes erupt from the ocean bottom, and trenches and valleys plunge to great depths.

Scientists called oceanographers work to discover the ocean's secrets. They study how the ocean moves and how it affects the atmosphere. They investigate the chemical composition of seawater and monitor the various forces that shape the sea floor. They also study how organisms live in the sea and how people have affected the ocean.