The Wadden Sea stretches from Den Helder, in the northwest of the Netherlands, past the great river estuaries of Germany to its northern boundary at Skallingen in Denmark along a total coastline of some and a total area of about . Within the Netherlands it is bounded from the IJsselmeer by the Afsluitdijk. Historically, the coastal regions were often subjected to large floods, resulting in thousands of deaths, including the Saint Marcellus' flood of 1219, Burchardi flood of 1634 and Christmas Flood of 1717. Some of these also significantly changed the coastline. Numerous dikes and several causeways have been built, and as a result recent floods have resulted in few or no fatalities (even if some dikes rarely and locally have been overrun in recent history). This makes it among the most human-altered habitats on the planet.