Barrier reefs are large coral formations that run parallel to coastlines, separated from the shore by a lagoon. They are typically found in tropical and subtropical waters and provide essential habitats for a diverse range of marine life. The structure of barrier reefs is built by tiny coral polyps, which secrete calcium carbonate, creating a solid foundation over time. One of the most famous examples of a barrier reef is the Great Barrier Reef in Australia, which is the largest coral reef system in the world. Barrier reefs play a crucial role in protecting coastlines from erosion and storm surges, while also supporting fishing and tourism industries.