Storegga submarine slides are enormous underwater movements of glacial sedimentary deposits that were only recently discovered. While many of these occurred following the melting of the glaciers at the end of the last Ice Age some more recent movements have also been identified. Either earthquakes or the decomposition of gas hydrates are their most likely cause. Two storegga have been identified that occurred off the west coast of Norway around 6000-5000 BC. The combined volume of material involved in the last two events has been calculated at 1700 cubic kilometres. As a consequence, gigantic tsunami were generated which flooded a number of landmasses including Scotland. Ulf Erlingsson suggests that one of these events, around 6100 BC, may have been responsible for the submergence of the Dogger Bank in the North Sea, creating the impassable muddy shoals mentioned by Plato.