The Inlaatduiker in Gruiterpolder bij Spaarndam is thought to have been built in the year 1890. It forms a part of a defence line which spans over 135KM around Amsterdam, and served to protect the city from invasion by flooding strategic points to make them impassable in times of war. The Inlaatduiker is a UNESCO protected monument, but is not presently open for visitors, although it is interesting to view it from the outside.
