The series of windmills at Kinderdijk have been put on the World Heritage List of UNESCO in 1997. The Alblasserwaard polders, the area to which Kinderdijk belongs, are situated below sea-level and around 1740 the 19 sturdy windmills at Kinderdijk have been built to drain the excess water from the polders, pump the water into a reservoir (boezem) until the level of the river had reached a level that pumping the water into the river was possible again. The mills have been well preserved to the present day.