The Wadden Sea is not a sea in the conventional sense but an extensive body of shallow water and associated tidal flats, salt marshes, and low-lying islands in northwestern Europe. Straddling the shores of Denmark, Germany, and the Netherlands, the Wadden Sea has been formed by storm surges and sea-level rise inundating an area of coast, combined with the deposition of fine silt by rivers. It is an important nursery for North Sea fish species such as plaice and common sole, and its extensive mudflats are home to enormous numbers of mollusks and worms. The salt marshes provide a habitat for more than 1,500 species of insects and are important feeding and breeding grounds for many species of birds. Unfortunately, these marshes are threatened by agriculture, dike building for coastal protection, and tourism.