The Eider barrage is a piece of engineering that most people drive across without noticing, which is a pity. It closes the mouth of the Eider river near Tönning, on the west coast of Schleswig-Holstein, with a line of huge steel gates that stay open for the tide and slam shut when a storm surge threatens. It was built in the years after the great North Sea flood of 1962, finished in the early 1970s, and it shortened the dike line along the coast by a considerable margin while keeping the Eider's estuary alive for ships and wildlife.
You can ride across it, and you should stop at either end. The road runs along the structure with wide water on both sides, wind shoving you gently around, and gulls hanging in the gale like kites. On the seaward side the Wadden Sea opens out, a landscape that changes by the hour: at low tide it is mud, sandbars and seals, at high tide it is grey water to the horizon. The whole area belongs to the Wadden Sea national park and is a UNESCO site, which explains the birdwatchers with long lenses you will see on the dike.
The riding in this region is very particular. Roads are flat, straight and exposed, with long views of dike lines, sheep and wind turbines turning in the same direction. Crosswinds are the one real hazard, and a gust on the barrage or an open stretch of road can nudge a light bike sideways. The weather is changeable, and rain can come in horizontally, so a waterproof layer should live in the pannier. Summer evenings, when the light goes long and yellow and the tourists have gone, are the best.
Practicalities are simple. The nearest proper town is Tönning, a few kilometres off, with fuel, a harbour with a pretty old timber crane, and plenty of fish sandwiches. Garding and Wesselburen are smaller but have supermarkets and cafés. Out on the barrage there is not much beyond a car park and a tiny visitor building, so bring a drink and layers, and take your time. A cold wind straight off the North Sea is part of the deal.