Reef caves are formed inside coral or sponge reefs by the growing reef. They are formed during the growth of the rock, at the same time the limestone is forms. The reef grow with different speed at different locations, so ridges and open hollows are formed. Sometimes the hollow parts are overgrown and enclosed, which results in a cave.
Reef caves are very popular with divers, cave diving as a sport is often limited to diving such reef caves. However, the underwater location means that these caves are not accessible to ordinary tourists. Many limestones originally form as reef, but undergo diagenesis as part of a tectonic subsidence, in which the reef is converted into limestone. The reef caves are usually not preserved as a cavity. They are filled with loose material or crystallize. Nevertheless, they later represent a zone of weakness along which secondary cave formation can begin.