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.
The islands in the Wadden Sea are called the Wadden Sea Islands or Frisian Islands, named after the Frisians. 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 Heligoland, although ethnically one of the Frisian Islands, is not situated in the Wadden Sea.
The word wad is Dutch for "mud flat" (Low German and German: Watt, Danish: Vade). The area is typified by extensive tidal mud flats, deeper tidal trenches (tidal creeks) and the islands that are contained within this, a region continually contested by land and sea. The landscape has been formed for a great part by storm tides in the 10th to 14th centuries, overflowing and carrying away former peat land behind the coastal dunes. The present islands are a remnant of the former coastal dunes.
The islands are marked by dunes and wide, sandy beaches towards the North Sea and a low, tidal coast towards the Wadden Sea. The impact of waves and currents, carrying away sediments, is slowly changing the layout of the islands. For example, the islands of Vlieland and Ameland have moved eastwards through the centuries, having lost land on one side and grown on the other. The Wadden Sea is famous for its rich flora and fauna, especially birds. Hundred of thousands of waders (shorebirds), ducks, and geese use the area as a migration stopover or wintering site, and it is also rich habitat for gulls and terns.
Each of three countries has designated Ramsar sites in the region (see Wadden Sea National Parks for the protected areas within the German borders). Although the Wadden Sea is not yet listed as a transboundary Ramsar site, a great part of the Wadden Sea is protected in cooperation of all three countries. The governments of the Netherlands, Denmark and Germany have been working together since 1978 on the protection and conservation of the Wadden Sea.
Co-operation covers management, monitoring and research, as well as political matters. Furthermore, in 1982, a Joint Declaration on the Protection of the Wadden Sea was agreed upon to co-ordinate activities and measures for the protection of the Wadden Sea. In 1997, a Trilateral Wadden Sea Plan was adopted.
Many of the islands have been popular seaside resorts since the 19th century. Mudflat hiking (Dutch: Wadlopen, German: Wattwandern), i.e., walking on the sandy flats at low tide, has become popular in the Wadden Sea. It is also a popular region for pleasure boating.