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 is rich in biological diversity. 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. The Wadden Sea’s coastline has been heavily modified by man. Extensive systems of dikes and causeways make it among the most human-altered on the planet.