A coral reef is composed of a calcereous exo-skeleton secreted by billions of tiny animals called polyps. Millions of polyps grow on top of the limestone remains of former colonies to create the massive reefs. It takes years for some corals to grow even one centimetre! They range in size from a pinhead to a few feet in length. Amazingly, these tiny animals form the only natural structure visible from space.
Each polyp lives in a symbiotic relationship with a type of algae, zooxanthellae, that gives the coral its colour.
Read more at: http://www.nature.org/joinanddonate/rescuereef/explore/facts.html