The Great Barrier Reef is the largest group of coral reefs in the world. The coral in the reef consists of limestone that is beneath the ocean or slightly above the water. The reef is known for its beauty. Its chain length of reefs extends about 1,250 miles along the northeast shore of Australia. Some parts of the reef are more than 100 miles from the coast. The closest part is 10 miles out. Millions of living polyps are stuck to the coral. These polyps are red, green, blue, purple, and yellow. Many small islands are throughout the reef. About 400 species of polyps live in the reef and about 1,500 species of fish live there. Other creatures that live in the reef are crabs, clams, and sea turtles. Some scientists believe that the Great Barrier Reef was formed 500,000 years ago.