The individual coral polyps are small and soft bodied with a hard external skeleton surrounding it. These small animals live in large colonies building off the skeletons of other polyps. While the coral is strong enough to be used as building blocks for houses it is very susceptible to some environmental factors such as acid rain. Most reefs are in the tropics, the area between the Tropic of Cancer and the Tropic of Capricorn.
Coral begins as an individual polyp the size of a pea.
They reproduce by creating new branches or sprouts. While the individual animals are small a colony can be huge. The Great Barrier Reef in Australia is the only living structure that can be seen from space. These structures made of living and dead skeletons are strong enough to withhold currents, salt water and temperature changes to name a few. Humans have used blocks made of coral for construction for many years.