The world's largest coral reef system, stretching over 2,300 kilometres off Australia's northeast coast, is the Great Barrier Reef.
It lies in the Coral Sea off Queensland and is made up of more than 2,900 individual reefs and about 900 islands and coral cays. Rather than being one continuous wall, it is a vast network of reef habitats separated by channels and open water.
The reef was built over immense periods by billions of tiny coral polyps. It supports thousands of marine species, including fish, sea turtles, sharks and dugongs, and can be seen from space. The Great Barrier Reef Marine Park protects much of the ecosystem.
A common mix-up is confusing the reef system with the Great Barrier Reef Marine Park, which is the protected management area around it. The reef itself is also described as the largest structure made by living organisms, although it is a biological system rather than a single solid object.