Barrier reefs also have a reef flat covered with ocean water during high tide, but has little or no water on it during low tide. Many corals grow on the outer reef front but they also grow well in the lagoon. The part of the reef that faces the lagoon is called the inner reef. The barrier reef acts as a wall that protects the lagoon and the inner reef. Boats often dock in the lagoon for protection against strong waves, but must be careful not to run into the reef where corals are growing in shallow water.
Atolls grow in the shape of a circle.
In the middle of a circle is a lagoon. Usually atolls are far away from any large islands or continents, but they often have narrow sandy beach shoreline and palm trees growing on them. Like on barrier reefs corals growth on atolls is best on the reef front and inner reef. It is believed that atolls are formed while a tropical island sinks slowly over thousands of years. The reef front of a fringing reef around an island actively grows at the same time the island is sinking.