The oceans

The ocean covers about 71 per cent of the Earth's surface. That is why the planet looks blue from space, and why early astronauts called it the 'blue planet'.

The oceans hold 97 per cent of the Earth's water (only about 3 per cent is fresh water), and support life from tiny plankton to the blue whale, the largest mammal in the world, over 30 metres long and weighing almost 150,000 kg.

They also regulate the climate. They absorb heat, release it slowly, and add moisture to the air, forming clouds and rain. And the ocean floor is not flat: it has mountains, valleys and plains, just like the land.

Here is a fact worth pausing on: the oceans give us almost half the oxygen we breathe, and it is plankton that makes this possible: microscopic living things with chlorophyll that use sunlight and carbon dioxide to make food.

So half of each breath you take came from creatures too small to see.

The five oceans (remember PAISA: Pacific, Atlantic, Indian, Southern, Arctic) each have their own character:

• Arctic: the smallest and shallowest, covered with sea ice, home to polar bears and walruses.
• Indian: warm waters, rich wildlife, and a long history of international trade.
• Atlantic: one of the busiest waterways, and a rich fishing ground.
• Pacific: the largest and deepest.
• Southern: circles Antarctica.

The oceans are also always moving. Ocean currents are like giant rivers flowing through the sea: warm currents carry water from the equator towards the poles, and cold currents carry it back.

Those currents affect the land too. The Gulf Stream carries warm water from near the Equator across the Atlantic to Western Europe, which is why the United Kingdom is warmer than other places at the same latitude.