Zuider Zee (zĪˈdər zē, zā, Du. zoiˈdər zā) [key], former shallow inlet of the North Sea, c.80 mi (130 km) long, indenting NE Netherlands. In ancient times Lake Flevo, it was joined to the North Sea by a great flood in the 13th cent. A vast drainage project, begun in 1920, split the old Zuider Zee into the IJsselmeer, S of the IJsselmeer dam, and the Waddenzee, between the dam and the West Frisian Islands. Much of the IJsselmeer has since been reclaimed for farm lands and urban expansion.