What is a coral reef?

Coral reefs are enormous underwater structures that provide food and shelter for nearly one quarter of all marine life in the world. They are home to 4,000 species of fish, 700 species of coral and millions of other plants and animals. Coral reefs are like huge, busy underwater cities, with thousands of underwater inhabitants rushing around them.

Coral reefs are built by tiny animals called corals that have lived on Earth since before the dinosaurs existed. Amazingly for such small creatures, the reefs they build can be hundreds of kilometres long and thousands of metres thick. The longest coral reef in the world - the Great Barrier Reef near Australia - can even be seen from space!