Glacial Floods

A big event occurred about 8200 years before the present when the Atlantic Ocean was flooded over a period of only one year by the breaking of the ice dam at the northeastern edge of North American at the end of the Ice Age, which caused the filling of the Irish Sea, the North Sea, the English Channel and the Baltic Sea. This was accompanied by much flooding in the valleys of Western Europe and much colder temperatures for about 300-500 years in western Europe. (The deeper sections would have been covered with water already.) Articles about this:
A map of Europe before the Atlantic flooding.

There was another big event at 7600 ybp, the flooding of the Black Sea area ( book page).