The Five Oceans

Most of the Earth is covered in water. In fact, about 71% of our planet's surface is ocean — that is why, seen from space, Earth looks like a beautiful blue marble. There are five oceans: the Pacific, Atlantic, Indian, Southern, and Arctic. Together they form one enormous, connected body of water that you could, in theory, sail right around.

Ocean water is salty because rivers carry tiny amounts of salt and minerals, washed out of rocks, down to the sea. When ocean water evaporates into the sky it leaves the salt behind, so over millions of years the sea has become saltier and saltier. This is different from the fresh water in most rivers and lakes, which you could safely drink.

Oceans are full of life, from tiny glowing plankton to the blue whale, the largest animal that has ever lived. The deepest parts are darker and colder than anywhere on land. Scientists divide the ocean into layers, or zones, depending on how much sunlight reaches them. Sunlight can only shine a little way down before everything turns to darkness.

Deep down in the ocean it is pitch black and freezing cold. Amazing creatures live there — some even make their own light to find food and attract a mate, a clever trick called bioluminescence. The anglerfish, for example, dangles a glowing lure in front of its mouth to tempt smaller fish closer.

The oceans are also home to coral reefs, which are built by tiny animals called coral polyps. Reefs are sometimes called the rainforests of the sea because so many different creatures live among them. Sadly, pollution and warming water can damage reefs, so looking after the ocean matters more than ever.