This large of a coral reef was taken by NOAA photographers in the waters off the coast of the U.S. Virgin Islands in the Caribbean Sea. Brain corals get their name from their spherical shape and grooved surface which resembles a brain. Each head of coral is formed by a colony of tiny polyps which secrete a hard skeleton of calcium carbonate. Brain corals are important contributors to the formation of coral reefs. Brain coral colonies can grow as large as 6 feet (1.8 meters) in diameter.