A substantial amount of prehistoric and historic cultural landscapes is now submerged. Sea levels have fluctuated throughout geological time, periodically encroaching or retreating across coastal plains. These now-submerged zones were important for prehistoric and historic humans, allowing access to marine and terrestrial resources and to transportation and migration routes. The principal process contributing to sea-level changes is the exchange of water between the continental ice sheets and the oceans (glacio-eustatic sea-level changes). Eustatic sea-level changes occur on an oceanic to worldwide scale. They also result from a change in the size of the ocean basin following tectonic seafloor spreading (tectono-eustasy) or sedimentation (sedimento-eustasy). In addition, sea level changes can be driven by local changes of the land with respect to the sea surface due to tectonic deformations, sediment compaction, and human activity. Inundated terrestrial archaeological sites, however, can result from a number of other natural processes, as well as sea-level changes, including earthquakes (such as Port Royal in Jamaica), volcanic processes (such as the ports of Misenum and Baiae, and Portus Julius and Nisida in the Bay of Naples) and flooding event (such as Herakleion and Eastern Canopus in Egypt).