A former large, landlocked inlet off the North Sea, on the N coast of the Netherlands. The sea washed into it only during floods. Once again it is landlocked by a dike, partly drained, and is divided into the IJsselmeer and the Waddenzee. As early as the 16th century plans were made to enclose the southern part of the sea. After the last break in 1916, a vast drainage project was begun, and a dam was completed in 1932, making the Dutch coastline shorter and easier to protect and providing for an increasing number of polders, or areas of drained land.