The physics of the place is beautifully lopsided. The tidal range grows as you travel south, from about 1.5 metres off the north coast of Sylt to 3.7 metres in southern Dithmarschen. Everywhere in the Wadden Sea the flood takes only about 85 per cent as long to come in as the ebb takes to go out, which means the incoming current runs harder than the outgoing one. Sediment arrives faster than it can be carried away, and so the flats build themselves, grain by grain, twice a day, forever. What the water does not fill in becomes channel: the Lister Tief, the Heverstrom, the Piep, deep tidal rivers scouring their way through the mud. Fed by the Rhine, the Elbe and the rest of northern Europe's plumbing, the water is brackish rather than properly oceanic, and the seabed slopes so gently that in places it drops a few centimetres per kilometre.

In spring and autumn the trilateral Wadden Sea holds around ten million migrating birds - more than any other place in Europe. The flats are a refuelling station on the East Atlantic flyway, and the calories come out of the mud: worms, snails, cockles, shrimp, harvested by bills built for the job. Eelgrass meadows feed barnacle geese. But the strangest gathering is a moult, not a migration. Every July through September some 180,000 shelduck converge on the flats around the uninhabited island of Trischen to shed their flight feathers all at once - over eighty per cent of the entire northwest European population, flightless together in one place. Nothing comparable happens anywhere else on Earth. Around 200,000 eider join the moulting crowds. Behind them, the salt marshes are flooded anywhere between ten and 250 times a year, and support roughly fifty species of flowering plant and two thousand species of insect specialised enough to survive being drowned in salt water on a schedule.

The sandbanks belong to the seals. A 2017 count put the harbour seal population of the Schleswig-Holstein Wadden Sea at around thirteen thousand, roughly a third of them born that same year - a recovery from the disease epidemics that gutted the population in living memory. Grey seals are far rarer, about 140 animals, concentrated on the Jungnamensand and the Knobsand banks off Amrum; they are the larger, longer-faced cousins, and they pup in winter while the harbour seals pup in summer. Offshore, harbour porpoises gather in unusual density, enough that a dedicated protection zone of 124,000 hectares was drawn west of Sylt in 1999. Roughly a third of the park is protection zone 1, where public access is largely closed; south of the Hindenburgdamm there is a zero-use zone of 12,500 hectares where humans are simply not part of the arrangement.

Scattered through the North Frisian flats are the Halligen - marsh islands with no dikes at all, the remnants of larger land masses that the great floods of 1362 and 1634 pulled apart. Instead of holding the sea out, the people living on them let it in. Houses stand clustered on Warften, artificial earth mounds raised above the marsh, and several times each winter the tide rises over the grass and the island disappears except for those mounds and the church. The locals call it Landunter. Roughly 230 people live on the German Halligen; Langeness, Hooge, Groede, Oland and Nordstrandischmoor are inhabited and carved out of the national park boundary, while smaller ones like Suedfall, Norderoog and Habel belong to the birds. Trischen, out in the Dithmarschen flats, is the only sandbank there high enough to grow salt-tolerant vegetation, and every attempt anyone made to fortify it has failed.

Protection was not a gift the coast asked for. Proposals in the 1970s collapsed under local resistance, and it was not until the National Park Act of 22 July 1985 that the park came into being on 1 October that year. Fishermen, dike shepherds and Hallig farmers, whose ancestors had made this coast habitable at considerable personal cost, saw the whole thing as rule from distant Kiel. When the park was enlarged in 1999, 143 shrimp boats sailed through the Kiel Canal to protest outside the state parliament, and at a public meeting in Toenning the environment minister Rainder Steenblock was pelted with eggs. It ended better than it began. UNESCO named the biosphere reserve in 1990 and inscribed the Wadden Sea as World Heritage on 26 June 2009. In 2016 some 143,000 people walked out onto the flats on 5,900 guided mudflat tours. Recent surveys find roughly 85 per cent of people in Dithmarschen and North Frisia would now vote to keep the park.