 Atoll lagoons form as coral reefs grow upwards while the islands that the reefs surround subside, until eventually only the reefs remain above sea level.Unlike the lagoons that form shoreward of fringing reefs, atoll lagoons often contain some deep (>20m) portions.Coastal lagoons form along gently sloping coasts where barrier islands or reefs can develop off-shore, and the sea-level is rising relative to the land along the shore (either because of an intrinsic rise in sea-level, or subsidence of the land along the coast).