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Coral reefs are built, layer upon layer, by millions of individual animals. These polyps build an intricate web of limestone on which a reef stretching many miles can be supported. Coral reefs are only found in the clear, shallow water of the tropics, where the warm water and sunlight can provide energy for algae growing inside the polyps. The algae can convert this sunlight into sugars, which is then used by the polyps as food.