The Wadden Sea is the intertidal wetland in the south-eastern corner of the North Sea. It runs for about 500 km along the coasts of the Netherlands, Germany and Denmark, from Den Helder in North Holland to Skallingen in south-west Jutland, and covers roughly 10,000 km². Most of it is mudflat and sandflat that the tide floods and uncovers twice a day, with deep tidal channels, salt marshes and a chain of low Frisian barrier islands on the seaward side. UNESCO inscribed the Dutch and German parts as a World Heritage Site in 2009 and added the Danish part in 2014. Germany and Denmark protect their parts as national parks, UNESCO declared the area a biosphere reserve in 1986, and all three countries have designated Ramsar sites along it. People have lived here for at least 2,500 years, and the dikes and causeways they built make it among the most human-altered habitats on the planet, with dwelling mounds, shrimp harbours and seaside resorts all along it.
Terps and Warften: 2,500 Years of Building Upwards
The first people to farm the salt marshes of the Wadden coast did not hold the sea back. They lifted themselves above it. Around 500 BC families in what are now the Dutch provinces of Friesland and Groningen began piling clay, dung and household waste into artificial dwelling mounds, called terpen in Friesland, wierden in Groningen and Warften or Wurten in Germany. A second building period followed between 200 and 50 BC. In the mid-3rd century AD the sea rose so sharply that the clay district was abandoned, and settlers did not return until around AD 400. A third terp-building era began about AD 700 and lasted until dikes arrived around 1200.
Some mounds grew to 15 m high, and about 1,200 survive in Groningen and Friesland alone. Many village names still give them away: places ending in -werd or -ward sit on a mound, and so do most villages ending in -um, from an old word for a farmyard. Centuries of rubbish made the mounds rich in nutrients. In the 18th and 19th centuries farmers dug many of them away to spread on their fields, which destroyed a large part of the archaeological record. One old terp, Het Torp, lies under the naval town of Den Helder at the western end of the sea.
The Storm Floods of 1362, 1634 and 1717
The Wadden landscape is younger than most of its villages. Between the 10th and 14th centuries, storm tides broke through the coastal dunes and carried away the peat land behind them. The present islands are what is left of that old dune line. The best-known flood came on 15 or 16 January 1362, although an earlier flood in 1219 may have drowned even more, some 36,000 people. The Grote Mandrenke, or "great drowning of men", killed an estimated 25,000 people, destroyed perhaps 30 settlements and pushed the North Frisian coastline eastwards. One of the lost places was Rungholt, a trading town on the island of Strand that may have had 500 houses and about 3,000 inhabitants. Ruins were found near Hallig Südfall in 1921, and in June 2023 the German Research Foundation announced that researchers had confirmed the identification. By then about 10 km² of the surrounding area had been mapped, including a church. Archaeologists blame drainage of the peat for the town's vulnerability: once the peat was cut and drained, the land sank lower behind its dikes.
On the night of 11–12 October 1634 the Burchardi flood finished off Strand. Between 8,000 and 15,000 people drowned. What was left of the island became Nordstrand, Pellworm and several small undyked islands. The Christmas Flood of 1717 struck the whole coast from the Netherlands to Denmark and killed about 14,000 people. Some 9,000 of them died between Tønder and Emden, and 2,500 in the Netherlands. Butjadingen lost 30% of its population and East Frisia lost 900 houses. These floods, and the dikes built after them, set the coastline that the Wadden communities live on today.
Hooge, Langeneß and the Undyked Halligen
The Halligen are the clearest picture of how people adapted to that sea. These small islands off Schleswig-Holstein have no protective sea dikes at all. Homes and farm buildings stand on metre-high Warften, and in winter storms the open meadows flood until each mound becomes an island of its own. Ten Halligen survive in Germany and one, Langli, in Denmark. Mandø, also in Denmark, used to be a hallig but now has dikes.
The largest is Langeneß, at 956 ha, with 16 Warften and about 110 inhabitants. Hooge covers 574 ha, with 10 Warften and about 120 people. Gröde has 11 residents. Nordstrandischmoor has 18, a one-room schoolhouse and a narrow-gauge railway to the mainland. Oland has about 30 people living on a single Warft, linked by rail and causeway to both the mainland and Langeneß. Süderoog has two residents, and Norderoog, Südfall and Habel are bird sanctuaries. People on the Halligen live mainly from tourism, coastal-protection work and cattle grazing on the fertile salt meadows. The five larger ones, Langeneß, Hooge, Gröde, Oland and Nordstrandischmoor, are surrounded by the Schleswig-Holstein Wadden Sea National Park but are not part of it.
Brown Shrimp and the Krabbenkutter Fleets
The main commercial catch from the shallow Wadden channels is the brown shrimp, Crangon crangon. It is a sandy-coloured crustacean, usually 3–5 cm long, that buries itself in sand by day and feeds at night. It is fished mainly in the southern North Sea, by small beam-trawl cutters known in German as Krabbenkutter, which work out of harbours along the Frisian and Dithmarschen coasts. More than 37,000 tonnes were caught in 1999, and German and Dutch boats landed over 80% of that total.
Other fisheries in the area have not survived. The European oyster once formed large beds in the Wadden Sea and was still present a few decades ago, before disease and the spread of the invasive Pacific oyster wiped it out. Pacific oysters now form large beds of their own. The south-western Wadden Sea once took most of its fresh water from the Rhine, which dams have reduced, and about 90% of the species that historically lived in that part are now at risk.
Esbjerg, Den Helder and the Causeway Towns
One of the largest towns on the Wadden Sea is also one of the youngest. Esbjerg, at the northern end, was founded by royal decree in 1868, after Denmark lost its main North Sea port at Altona in the Second Schleswig War of 1864. The harbour opened in 1874 with rail links to Varde and Fredericia. It grew from about 400 inhabitants in the early 1870s to 70,000 by 1970. On 1 January 2026 its urban population was 71,371, making it Denmark's fifth-largest city. Its harbour, the second largest in Denmark, grew on fishing and farm exports. A ferry to Harwich in England ran from 1875 until September 2014, and the port is now expanding for the offshore wind industry.
At the opposite end, Den Helder looks across the Marsdiep channel to Texel. Texel is the largest and most populous of the West Frisian Islands, with 13,846 residents. The All Saints' Flood of 1170 cut it off from North Holland, and it received city rights in 1415. East of Den Helder, the 32 km Afsluitdijk was built between 1927 and 1932 from Den Oever to Zurich in Friesland. It sealed the Zuiderzee off from the sea, turned it into the freshwater IJsselmeer and fixed the Wadden Sea's Dutch boundary. Since 2019 the dike has been undergoing a major upgrade that will raise it by about 2 m.
Sylt, Germany's largest North Sea island at 99.14 km², has been an island only since the flood of 1362. Since 1 June 1927 it has been joined to the mainland by the Hindenburgdamm, an 11 km railway-only causeway. More than 100 trains a day cross it, about 50 of them carrying cars, and together they move over 450,000 vehicles a year. The causeway has also blocked the tidal flow between Sylt and the mainland, and that change is thought to be one cause of land loss at the island's southern tip.
Wadlopen, Wattwandern and the Island Resorts
Many of the islands have been seaside resorts since the 19th century, and tourism is now the main income for much of the coast. One activity is unique to the region: walking across the seabed at low tide. In the Netherlands this is wadlopen. It is allowed only on recognised routes, with a licence or a licensed guide, and guiding organisations are based at Pieterburen, Holwerd and Uithuizen. Walkers can cross from the Frisian mainland to Terschelling, Ameland, Schiermonnikoog, Engelsmanplaat and Rottumeroog. In Germany, routes lead to Norderney, Baltrum, Langeoog, Spiekeroog and Neuwerk, and to several Halligen and Föhr. The crossing between Amrum and Föhr requires a guide. Danish walkers go to Mandø, Fanø and Langli. Wattwandern is banned near the Hindenburgdamm, which lies in the strictly protected Zone I of the national park.
Trilateral Rules from 1978 to the 2014 World Heritage Extension
Because the people who use the Wadden Sea live in three countries, protecting it is a joint job. The Netherlands, Germany and Denmark have cooperated on its protection since 1978. They signed a Joint Declaration on the Protection of the Wadden Sea in 1982 and adopted a Trilateral Wadden Sea Plan in 1997, coordinated from the Common Wadden Sea Secretariat in Wilhelmshaven. The Schleswig-Holstein Wadden Sea National Park was founded on 1 October 1985 and is Germany's largest national park, at 4,410 km². Lower Saxony followed in 1986 with a park of about 3,458 km² running from the Dollart bay to Cuxhaven. In Denmark, Nationalpark Vadehavet, designated in 2008, covers the Danish Wadden Sea from Ho Bugt to the German border, including Fanø, Mandø and Rømø. Hamburg's park joined the World Heritage Site in 2011. The state of Bremen, on the Weser estuary, does not take part.
The rules shape everyday work on the coast, from where cutters may fish to which flats walkers may cross. Sea-level rise is the long-term worry. Recent studies suggest that sediment is building up on most tidal flats faster than the current rise of 3.7 mm a year, especially on the German coast, while the Dutch flats are keeping up more slowly. For the wider basin these flats drain into, see our guide to the North Sea. For the Dutch lowlands behind the Afsluitdijk and the terp country, read about the geography of the Netherlands. And for the river whose estuary marks the edge of Hamburg's Wadden park, follow the Elbe from its source to Cuxhaven.