What we call "hard or stony corals" are actually skeletons of small
marine animals called coral polyps, which grow in groups or "colonies".
As polyps die, new ones grow on top of their skeletons, very slowly
building coral reefs. With some species needing 250 years to build one
meter of coral reef, it is believed that some of the world's largest
reefs started forming more than 400 million years ago.