Caves under the sea or partially submerged sea caves are distributed widely throughout the SAC in areas of suitable rock. The predominantly rocky coastline within the site is geologically complex, in both the range of rock types and the complexity of faulting and folding, and is highly predisposed to sea cave formation. Partially submerged, intertidal caves are widely distributed throughout the length of the rocky coast, with the largest known concentrations on St David’s peninsula, Ramsey Island, Skomer Island and the Castlemartin coast. The distribution of submerged sea caves is poorly known; the few that have been documented have been the result of opportunistic discovery. Individual caves range in size from very small (little more than deep, enclosed overhangs) to extremely large; either very long (e.g. Ramsey Island, > 50 metres) or high and wide (e.g. South Pembrokeshire limestone).

Sea caves typically support species that are “out of place” because caves provide areas which are physically different (e.g. darker, more sheltered) than immediately outside the cave (e.g. sponges typical of deep water in intertidal caves; mud dwelling anemones in sediments on the floor of caves in exposed rocky areas). Cave species differ according to the rock type of the cave. They often need to be very tolerant of scour and of extreme wave surge.