Coral reefs are ecosystems characterized by reef-building colonies of coral. These corals build elaborate and colorful calcium carbonate architectures that in turn become homes to a wide range of aquatic life. Fictional coral reefs often contain shipwrecks, sunken ruins, sunken statues, or undersea habitation units. They may be home to underwater civilizations. They may be near kelp forests, underwater cave systems, or sudden oceanic drop-offs.