The Bermuda Platform is the elliptical, flattened summit of a huge volcanic submarine mountain (seamount) in the northwest Atlantic. Its surface lies 45–60 ft (14–18 m) below sea level and is covered in a thick layer of limestone, formed over millions of years from the remains of corals and other organisms growing on the platform. Along the platform’s southern and eastern edges, limestone sand has gradually built up to form the Bermuda islands. Coral reefs are present around the other edges of the platform, forming an atoll, while patch reefs grow on its central surface. The diversity of reef flora and fauna here is less than that associated with the reefs in the Caribbean Sea to the south. Nevertheless, 21 different species of stony coral, 17 species of soft (non-reef-building) coral, including many spectacular purple sea fans, and about 120 different species of fish have been recorded here.