The southeastern edge of the North Sea, along the coast from Denmark through to the Netherlands, is a shallow belt of mudflats and barrier islands. Usually, it remains underwater, but twice a day when the tide goes out, the waters retreat a massive 15 km revealing a huge expanse of tidal flats. This region is known as the Wadden Sea and it’s described as "one of the last remaining natural large-scale intertidal ecosystems" on earth.

The Wadden Sea stretches from Den Helder in the Netherlands in the northwest, past the great river estuaries of Germany to its northern boundary at Skallingen north of Esbjerg in Denmark along a total length of some 500 km and a total area of about 10,000 square km.

A group of mudflat hikers on the Wadden Sea near Pieterburen, Netherlands. Photo credit: Wikimedia