The Wadden Sea is an intertidal zone in the southeastern part of the North Sea. It lies between the coast of northwestern continental Europe and the range of low-lying Frisian Islands, forming a shallow body of water with tidal flats and wetlands. It has a high biological diversity and is an important area for both breeding and migrating birds. In 2009, the Dutch and German parts of the Wadden Sea were inscribed on UNESCO's World Heritage List and the Danish part was added in June 2014.
The Wadden Sea stretches from Den Helder, in the northwest of the Netherlands, past the great river estuaries of Germany to its northern boundary at Skallingen in Denmark along a total coastline of some 500 km and a total area of about 10,000 km². Within the Netherlands it is bounded from the IJsselmeer by the Afsluitdijk. Historically, the coastal regions were often subjected to large floods, resulting in thousands of deaths, including the Saint Marcellus' flood of 1219, Burchardi flood of 1634 and Christmas Flood of 1717. Some of these also significantly changed the coastline.