More than two-thirds of the Earth’s surface is covered by the waters of the oceans. About 97% of the Earth’s water is found in the oceans. There are five oceans. In order of size, they are: the Pacific, Atlantic, Indian, Arctic and Southern. We often use the word "sea" to mean ocean, although seas are really smaller parts of the ocean partly enclosed by land—for example the Mediterranean Sea or North Sea.

The ocean waters move around the globe in huge swirling currents. Surface currents are driven by the winds. They flow in roughly circular patterns: clockwise in the northern hemisphere and anticlockwise in the southern hemisphere. Some currents are warm while others are cold. They have a major effect on the climates of lands bordering the ocean. This is because the winds above the ocean are warmed (or cooled) by the waters below.

Beneath the ocean waters, there are vast plains and long mountain ranges. Skirting the land, there is a ledge called the continental shelf. From here, the ocean bed slopes steeply down to the abyssal plain. A long, jagged ridge called the Mid-Oceanic Ridge runs across of the abyssal plain. There are also deep gashes in the plain, called ocean trenches. The plain is also dotted with mountains. Some high peaks poke up above the waters to form islands.

Black smokers

In some places along the Mid-Oceanic Ridge, hot water jets up through tall “chimneys”. These are called black smokers. The water has come from inside the rocks below the ocean floor. The water is heated by magma (molten rock) and shoots up through cracks in the ocean floor. Strange animals live close to the black smokers. Among them are giant red worms that live in white tube shells.