A reef is a underwater structure made up of coral, rock, or sand that provides habitat for various marine life. Coral reefs, in particular, are formed by tiny animals called corals, which build calcium carbonate skeletons. These ecosystems are rich in biodiversity and are often referred to as the "rainforests of the sea" due to the variety of species they support. Reefs can be found in warm, shallow waters around the world, especially in tropical regions. They play a crucial role in protecting coastlines from erosion and provide resources for fishing and tourism. Additionally, reefs are sensitive to environmental changes, making them important indicators of ocean health.