The Great Barrier Reef off the north-east coast of Australia is regarded as one of the Seven Natural Wonders of the World.
It's home to thousands of different types of coral, plants, fish, and mammals.
But scientists says 50 percent of the coral has been destroyed over the past three decades because of a number of factors including pollution, fishing and boats.
The United Nations says the World Heritage Site may soon be listed as endangered.
The UN is especially concerned about the growth of coal mining along the Queensland coast, near to the reef.
The BBC's Australia correspondent Jon Donnison has been looking at debate surrounding the Great Barrier Reef.