Photograph by Raul Touzon, National Geographic
Coral reefs are complex structures built by tiny organisms called coral polyps, which are kin to jellyfish and sea anemones. Polyps attach themselves to sunken rocks at the edges of islands or continents. Their limestone skeletons connect with one another in massive numbers to create first coral colonies, then larger reefs—Earth’s largest biological structures. A healthy coral reef can live for many thousands of years.