Schleswig-Holstein Wadden Sea National Park

Listen first. Under the wind, there is a soft clicking and popping all around you. That sound comes from the mud itself. As the tide pulls back, millions of tiny creatures shift, breathe and burrow, and air escapes through countless small holes. The ground is grey and shining. It smells of salt, iodine and something faintly sulphurous. Here on Pellworm, the sea does not stay where you expect it. Twice a day, it simply walks away.

What you are looking at is the Wadden Sea. It is a vast band of tidal flats, channels and sandbanks along the North Sea coast of the Netherlands, Germany and Denmark. The section around you is protected as the Schleswig-Holstein Wadden Sea National Park. Germany created the park in 1985. In 2009, the Wadden Sea was added to the UNESCO World Heritage list, and the Danish part followed in 2014. It counts as one of the largest unbroken systems of tidal flats on Earth.

The rhythm here is simple. Roughly every six hours, the tide turns. At high water, boats move over the place where you could walk a few hours later. At low water, the sea floor lies open for kilometres, cut by winding channels the locals call priele. These channels never fully drain. They fill first when the tide returns, and they fill fast. That is the first lesson of this landscape — the flats look calm, but they change quickly.

Now look down at the mud near the shore. You will probably see small coiled heaps, like spaghetti made of sand. These are the casts of the lugworm. The worm lives in a U-shaped tunnel and swallows sand, digests the tiny organisms in it, and pushes the clean sand back out. A single square metre of these flats can hold huge numbers of worms, snails, shellfish and shrimp. This hidden life is the food supply for everything else.

And the biggest beneficiaries are birds. Each spring and autumn, millions of migratory birds stop in the Wadden Sea. Some fly between the Arctic and Africa, and this coast is their refuelling station. You might see dark brent geese grazing in tight flocks. You will almost certainly hear oystercatchers — black and white, with bright orange beaks and a loud, piping call. When a large flock of small waders lifts at once, the whole sky seems to turn and flicker.

Out on the distant sandbanks, there are other residents. Harbour seals rest there at low tide, and grey seals live in this part of the North Sea too. From the shore they often look like smooth grey stones, until one of them lifts its head. Summer is the season for harbour seal pups. If you ever find a young seal alone on a beach, do not touch it. The mother is often feeding nearby and may return.

Turn around and look at the island behind you. Pellworm sits low — much of it lies below the level of a high spring tide. A green dike runs around the whole island and holds the sea back. Without that wall of earth and grass, the fields, farms and houses you see would flood. Sheep graze on the dikes, and their hooves press the soil firm. They are working animals here, part of the coastal defence.

This island is itself the survivor of a disaster. Until the seventeenth century, Pellworm was part of a much larger island called Strand. In October 1634, a great storm flood broke through the dikes. Thousands of people died, and the island was torn apart. Two main pieces survived — Pellworm, and Nordstrand to the east. The sea between them is the place where whole parishes once stood.

There is an older story too, and here history and legend mix. Locals tell of Rungholt, a wealthy trading town that sank beneath the waves in a terrible storm flood in the fourteenth century. Legend says the people grew proud and godless, and the sea punished them. That part is folklore. But Rungholt was real. Researchers have found traces of old settlement and farmland in the flats not far from here, still hidden under the mud.

On a clear day, look across the water for low shapes on the horizon. These are the Halligen — small islands without a full dike. Their houses stand on raised artificial mounds called Warften. When a storm flood arrives, the meadows go under and the houses stand alone in the sea like ships. The people who live there accept this. Some of these islands lie inside the national park, close to Pellworm.

Along the edge of the dike, you will notice a strip of different green. This is the salt marsh. Only a few plants can survive regular flooding with seawater. One of them is glasswort, sometimes called samphire, with fleshy stems that taste of salt. In late summer, sea lavender covers parts of the marsh in a soft violet colour. These marshes are a nursery and a resting place, so much of them is closed to walkers.

If the open flats tempt you, walk with care. Short steps near the shore are fine, but going far out alone is dangerous. Fog can come in quickly, and the returning tide moves faster than a person can walk through deep mud. The safe way is a guided mudflat walk with a certified local guide. They know the channels, the tide tables, and where the ground is soft. They will also show you creatures you would never find yourself.

The national park has rules that protect the quietest zones. Some areas are completely closed to people, especially where birds breed or seals rest. The idea is simple. Humans may visit, but the tide and the animals set the schedule. Many visitors are surprised to learn that this park has almost no buildings and no fences. Its borders are mostly lines on a map, drawn across water and mud.

Before you leave, stand still for one more minute. Notice where the water is right now — coming in, or going out. Notice the light on the wet mud, and how it changes when a cloud passes. In a few hours, this view will look completely different. The flats will be a sea, or the sea will be flats. And the clicking under your feet will keep going, long after you have walked back over the dike.