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Coral are marine animals found in tropical ocean areas. They are best known for building large coral reefs. Like other cnidarians, a coral is not a single organism, but made up of thousands of small polyps. Each polyp measuring only a few millimeters in diameter. Coral catch and feed on organisms, such as small fish and plankton, using stinging cells on their tentacles. Reefs are formed over thousands of years from coral skeletons held together by layers of calcium carbonate produced by the coral. It takes a lot of polyps to form a reef, but over time reefs can grow very large. The Great Barrier Reef, after 500,000 years, is so large that it can be seen from outer space. It is the world's biggest single structure made by living organisms. Play the following videos to learn more about the coral.