Four concrete mega-structures in the old sea dyke near Ouwerkerk in Schouwen-Duiveland, the Phoenix caissons, accommodate the unique collection of the recently renovated Watersnoodmuseum. It was not until nine months after the flood that the last breach in the dykes of the south-westerly part of the Netherlands was closed off with these caissons. This historic site, with these silent witnesses of the flood and the following reconstruction, and the surrounding area were declared a National Monument fifty years later.