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. Coral reefs provide home to ~25% of all marine species. In fictional worlds, coral reefs may contain shipwrecks, sunken ruins, sunken statues, or undersea habitation units. They may be come to underwater civilizations. They may be near kelp forests or sudden oceanic drop-offs. Alien worlds may have their own version of coral reefs.