The Dutch landscape is defined by its extensive lowlands, as Jan van Goyen painted here. Much of the countryside consists of polders. The water level inside these areas surrounded by dikes is regulated with the help of mills. Schiphol Airport, too, is located in such a polder, approximately four metres below sea level. Before it was empoldered in the 19th century there was a large body of water here called the Haarlemmermeer.