the country to the North Sea, thereby creating the Zuiderzee. It in itself would cause much trouble until the building of the Afsluitdijk in 1933. Several storms starting in 1219 created the Dollart from the mouth of the river Ems. By 1520 the Dollart had reached its largest area. Reiderland, containing several towns and villages, was lost. Much of this land was later reclaimed. In 1421 the St. Elizabeth's flood caused the loss of De Grote Waard in the southwest of the country. Particularly the digging of peat near the dike for salt production and neglect because of a civil war caused dikes to fail, which created the Biesbosch, now a valued nature reserve. The more recent floodings of 1916 and 1953 gave rise to building the Afsluitdijk and Deltaworks respectively. Flooding as military defense The deliberate inundating of certain areas can allow a military defensive line to be created. In case of an advancing enemy army, the area was to be inundated with about 30 cm (1 ft) of water, too shallow for boats but deep enough to make advance on foot difficult by hiding underwater obstacles such as canals, ditches, and purpose-built traps. Dikes crossing the flooded area and other strategic points were to be protected by fortifications. The system proved successful on the Hollandic Water Line in rampjaar 1672 during the Third Anglo-Dutch War but was overcome in 1795 because of heavy frost. It was also used with the Stelling van Amsterdam, the Grebbe line and the IJssel Line. The advent of heavier artillery and especially airplanes have made that strategy largely obsolete. Modern developments Technological development in the 20th century meant that larger projects could be undertaken to further improve the safety against flooding and to reclaim large areas of land. The most important are the Zuiderzee Works and the Delta Works. By the end of the 20th century all sea inlets have been closed off from the sea by dams and barriers. Only the Westerschelde needs to remain open for shipping access to the port of Antwerp. Plans to reclaim parts of the Wadden Sea and the Markermeer were eventually called off because of the ecological and recreational values of these waters. Zuiderzee Works The Zuiderzee Works (Zuiderzeewerken) are a system of dams, land reclamation, and water drainage works. The basis of the project was the damming off of the Zuiderzee, a large shallow inlet