The coastline humans knew was often somewhere else
For more than 90 percent of human existence, UNESCO notes that global sea level was roughly 40–130 metres below today's level during long intervals. Continental shelves that are now seabed included rivers, wetlands, caves, hunting grounds and migration corridors.
Doggerland is the best-known European example
The southern North Sea once connected Britain to continental Europe. Sediments, peat, pollen, animal remains and archaeological finds reveal a changing Mesolithic landscape progressively fragmented by early-Holocene sea-level rise.
The global archaeological archive is enormous but patchy
Submerged prehistoric evidence exists around many continental margins. Ports, coastal dwellings, caves and whole palaeolandscapes can survive, but erosion, sediment movement, trawling, dredging and development affect preservation and visibility.
We cannot yet count the lost settlements
Bathymetric reconstruction can estimate former land area, but it cannot tell us how densely people occupied it. Claims about unknown advanced coastal civilizations require archaeological evidence from actual sites, not sea-level maps alone.
Research record
UNESCO — Underwater Cultural Heritage and understanding climate changeOfficial · open sourceUNESCO — 20 years of enhancing protection of underwater cultural heritageOfficial · open sourceUNESCO — Threats to Underwater Cultural HeritageOfficial · open sourceHow Much Prehistoric Landscape Lies Beneath Today’s Seas?
Direct answer: Vast areas of former habitable land now lie beneath shallow seas, including entire prehistoric landscapes. We know only a fraction of their archaeological record, and the amount of preserved settlement evidence remains uncertain.
Lost landscapes are established geology and archaeology. The evidence strongly supports more submerged-prehistory research but does not, by itself, support claims of lost advanced civilizations.