Anchialine caves contain water bodies that have a subterranean connection to the sea and no or little direct connection to the atmosphere. These ecosystems are structured by strong salinity gradients. A primarily marine community influenced by tides and currents at the cave entrance transitioning to a more insular and specialised biota in still brackish waters influenced by rainfall percolation deep in the landward section of the cave. They occur where carbonate rocks and lava tubes meet coastlines worldwide.