Cleopatra VII's royal quarters were inundated by earthquakes and tsunami, leading to gradual subsidence in the 4th century AD. This underwater section, containing many of the most interesting sections of the Hellenistic city, including the palace quarter, was explored in 1992 and is still being extensively investigated by the French underwater archaeologist Franck Goddio and his team.
Accepted as basically correct, his explanation involved considering that several tropical island types—from high volcanic island, through barrier reef island, to atoll—represented a sequence of gradual subsidence of what started as an oceanic volcano.
Most sinkholes in the county are formed by gradual subsidence of material into the hole rather than a sudden collapse.
Guyots show evidence of having once been above the surface, with gradual subsidence through stages from fringed reefed mountain, coral atoll, and finally a flat-topped submerged mountain.