The islands in the atolls of the Maldives rest on the shelf provided by the reefs. Many reefs have no islands at all, but all islands in the Maldives have an underlying coralline reef. Usually islands are flat and sandy. Often there is a rocky bottom made up of a coral rock conglomerate underneath the island proper. Some islands are long and sandy while others have a rounder shape. The round islands often are lower in the centre, sometimes having a muddy or marshy spot.