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Below the clear blue of these tropical waters lie living structures, structures of striking beauty and size.

These are coral reefs. Some rise like underwater mountains, solid forms as high as ten storey buildings.

The reefs are alive and growing. They're made by living creatures called coral, millions of tiny animals which live in hard limestone houses of different shapes, sizes and colours.

Growing from the seabed up to the surface, the corals group together forming the living reefs. Some corals grow on old reefs, mounds made from the skeletons of corals that have died over the years.

Corals need light from the sun to grow but most of the ocean is too deep for sunlight to reach the bottom.

This is why the corals grow on the Continental Shelf, the shallow seabed which surrounds Australia. The waters of the Shelf in northern Queensland are clearer than most coastal waters and the sun enters easily.

And in this part of Australia it's warm all year round.

But this hasn't always been the case.

Twenty five million years ago Australia was much further south in colder latitudes than it is today but slowly it drifted north and two million years ago the North Queensland coast lodged in the warm waters of what is now the Coral Sea.

The conditions were ideal for corals to grow. Since then the level of the sea has changed. At times it's dropped below the Continental Shelf and, without water, whole reefs have died but when the seas have risen new corals have grown.

The latest growth started about 8000 years ago when the sea settled at its present level. All of the reefs have a similar structure. They have an outer wall of coral which protects a flat inner section.

The outer section which faces the open seas has to deal with strong waves. As a result the corals grow high and strong building themselves up like barriers to take the force.

Despite the turbulence corals thrive. The water is clear and there's plenty of food.

The inner section of the reef is called the reef flat. Here there's less food and sand stirred up by the waves clouds the water. It can be harder for corals to grow so they fan out to catch the sun.

The reefs support the richest variety of marine life in the world. Millions of different types of plants and animals thrive in the warm clear waters.

The largest fish on the reef is the whale shark which grows up to eighteen metres long.

Butterfly fish have specially developed pointy mouths which help them feed in the small nooks and crannies of the sharp coral.

The coral is an ideal place for fish of all sizes to find shelter.

The Great Barrier Reef has survived, continuing to grow and change, for thousands of years.

But it's a fragile environment and it's been recognised as an area which needs protection.

In 1975, the Great Barrier Reef became a Marine Park which prevented oil exploration in the area.

In 1983, the whole reef system was made a world heritage area.

It's taken its place as one of the most prized possessions on earth and it's the only sign of life on this planet that is large enough to be seen from the moon, a massive living structure made by tiny animals.
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