If just one coral larva floating in the sea finds a suitable place to settle, it may begin to grow and form a new reef. In just a few days the larva begins to change into a polyp, similar to a sea anemone. It then starts to bud off, producing identical copies of itself and forming a new colony. Each small polyp forms a hard outer layer for protection and begins to grow, increasing in length an impressive 15cm a year. A mature reef can be thousands of years old.