The windmills of Kinderdijk.
Around 1740 there where build 19 windmills to prevent the Alblasserwaard polder from flooding. They had to pomp the water out of the river into a reservoir, when the river almost started to flood, and when the water was low they pomped it back again into the river. The windmills of Kinderdijk are since 1997 on the list of the UNESCO World heritage site. So if you’re going to visite the Netherlands one day this is one of the sites you have to visit. The Windmills of Kinderdijk.