Reefs form under water and can be made of coral, sand or rock. The top of the reef sits about two or three feet below sea level. Coral reefs are found in warm, shallow, clean water and the Great Barrier Reef is the largest coral reef of all. It (25) _____ for two thousand kilometres and forms a natural breakwater that protects the coast of Australia from the strong waves of the South Pacific. There are (26) ___ islands on the Great Barrier Reef formed by piled up coral and sand. They are, (27) ________, very difficult to reach as sharp coral will tear the bottom out of a boat. One must be careful not to step on living coral as it grows very slowly, about five centimetres a year. The Great Barrier Reef is (28) _______ to many types of fish. There are also other aquatic species (29) _______ depend on coral to protect them.