A coral reef is a large underwater structure made of dead and living corals (press to see more). In most healthy reefs, stony corals are predominant. They are built from colonial polyps from the phylum Cnidaria which secrete an exoskeleton of calcium carbonate. The reefs are formed in tropical marine areas (30 degrees north and south of the equator) and between the tropics. The reef acts as the home of many tropical fish and other animals. Coral reefs systems are a major tourism attraction because of their beauty and color of the corals and their many associated animals. 