Colonies of coral appear vibrant blue green in this Moderate Resolution Imaging Spectroradiometer (MODIS) image of Australia's Great Barrier Reef. Stretching over 2000 kilometers along the northeast coast of Australia, the Great Barrier Reef is the largest coral system that has ever existed. It is made up of about 2,900 reefs with at least 350 different species of coral, though only about six percent are coral reefs. Underlying sediments, twice as old as the reef itself, indicate that the region was once above sea level. Geological evidence shows that the reef began growing more than 25 million years ago. As the image shows, the "reef" is in fact composed of many individual detached reefs, separated by deep water channels. The calcareous remains of tiny creatures called coral polyps and hydrocorals provide the basic building material for the reefs while the remains of coraline algae and organisms called polyzoas provide the cement that holds the structure together. When fossilized, such reefs and the debris eroded from them form thick limestone units. Teaming with life, the region is home to a wide variety of marine animals including over 1500 species of fish, sea snakes, turtles, birds, mollusks, and dugongs. Broad Sound cuts into the Queenland coastline towards the southern edge of the reef in this image.
The Great Barrier Reef is the largest reef on Earth at the present day. The reasons for its size and longevity are the very stable geological setting of the Australian platform, and the favorable oceanic circulation. Coral cannot exist at temperatures below 21° centigrade (70 ° Fahrenheit). The warmth of the waters of the Australian continental shelf varies little with depth because of the stirring action of the southeast trade winds. These winds pound the outer edge of the reef for nine months of the year, and this also keeps the reef supplied with seawater rich in the organic material needed by the growing coral.