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 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 is one of the world's seas whose coastline has been most modified by humans, via systems of dikes and causeways on the mainland and low-lying coastal islands. The Wadden Sea stretches from Den Helder in the Netherlands in the northwest, past the great river estuaries of Germany to its northern boundary at Skallingen north of Esbjerg in Denmark along a total length 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 islands in the Wadden Sea are called the Wadden Sea Islands or Frisian Islands, named after the Frisians. These are remnants of the once expansive and now submerged Doggerland. However, on the westernmost Dutch island, Texel, the Frisian language has not been spoken for centuries. The Danish Wadden Sea Islands have never been inhabited by Frisians. The outlying German island of Helgoland, although ethnically one of the Frisian Islands, is not situated in the Wadden Sea.
The German part of the Wadden Sea was the setting for the 1903 Erskine Childers novel The Riddle of the Sands.