The Great Barrier Reef extends over 2,000 km, and has been built by tiny animals called coral polyps. Some of the Great Barrier Reef’s coral “skeleton” deposits date over half a million years old. The individual coral polyps that comprise the reef grow very slowly, increasing by only 1-3 cm a year. A cultural and ecological icon, the Great Barrier Reef has been visited by Aboriginal Australians for over 40,000 years and today attracts over two million tourists annually. Unfortunately the fragility of the reef’s ecosystem is now threatened by the effects of climate change on the temperature of the water in which it sits: the Coral Sea. Over the last decade sea pollution caused by farm runoff has caused coral bleaching, thus diminishing the appearance of one of the world’s greatest sights. The ecological damage also threatens those endemic creatures that rely upon the Great Barrier Reef for food and/or shelter. Many of these are themselves endangered species. The Great Barrier Reef is in fact a system of over 3,000 reefs and islands. The northern section of the reef contains deltaic and ribbon reefs. The most common occurrences of fringing and lagoonal reefs are in the southern sections of the reef. In the middle section you are most likely to find crescentic reefs, although this type is also found in the northern reef.