The Great Barrier Reef, when the tide is low, some coral reefs emerge to form coral islands. The scenery is charming, steep and unpredictable, and the currents are unusually complex. There are more than 400 different types of coral reefs, including the world's largest coral reef. There are 1,500 species of fish, more than 4,000 mollusks, 242 clustered birds and unique scientific research conditions. It is also home to some endangered animal species such as dugongs and giant green turtles.