Zuiderzee, former inlet of the North Sea. From the 13th to the 20th century, the Zuiderzee penetrated the Netherlands and occupied some 2,000 square miles (5,000 square km); it was separated from the North Sea by an arc of former sandflats that are now the West Frisian Islands. From about 400 ce these low-lying sandflats were inhabited by the Frisians, who in the face of rising sea levels built the first seaworks—dikes and terpen (or werden), mounds to which they retreated during periods of high water. The volume of these terpen ranks them among the great engineering works of humankind.

The territory that subsequently became the Zuiderzee was in the 1st century ce a mixture of lowland and freshwater lakes; central and largest of the lakes was called Flevo Lacus by the Romans. These lakes did not connect directly with the sea but emptied by way of a branch of the Rhine River. Later, however, during a period of rising sea level (250–600 ce), the river and the central lake were enlarged. A period of lower sea levels followed, but in the 13th century, notably during 1219 and 1282, further flooding submerged wide areas and created the Zuiderzee proper.

By about 1000 ce, however, the terpen area had been completely enclosed by dikes. The controlling of water levels within the dikes developed into the practice of reclaiming tracts of lowland from a body of water (see polder). By 1667 the making of polders had developed to the point that the damming of the Zuiderzee was proposed. A feasible method, however, was not forthcoming until the flood of 1916 hastened the adoption of a plan developed by Cornelis Lely. In 1927–32 a dam 19 miles (30 km) long, known as the Afsluitdijk (“Enclosing Dam”), was built across the Zuiderzee, separating it into the outer Waddenzee (open to the North Sea) and the inner IJsselmeer (Lake IJssel). By the early 1980s four polders, largely agricultural land, had been created through an elaborately constructed system of pumping stations, dikes, sluices, and locks. Nearly half the IJsselmeer of the 1920s (626 square miles [1,620 square km] of a total 1,328 square miles [3,440 square km]) has been reclaimed and the much-reduced IJsselmeer has gradually become fresh water. The completion of a proposed fifth polder, Markerwaard, was abandoned in the 1980s. See also IJsselmeer Polders.