60 Percent of The Netherlands lies below sea level and many pieces of it is ‘man-made’. In the beginning of this era man started to cultivate higher grounds and small pieces of swampland. This was done by draining the swamps with small ditches who lead the excessive water to the sea or a river. The then remaining dryer land was sufficient for farming. However, when the tide was extremely high the land flooded and they have to start all over again. Even in the 11th century the locals started to built dikes to protect their cultivated grounds. Also with dikes critical sea-arms were closed. This especially was done in the parts of Holland, with a higher population. Most parts of Amsterdam and Rotterdam was water. When the Dutch improved their technique for building dikes they started to dry many lakes in the 16th and 17th century. This was done by building a surrounding dike and the water was drained with water mills and ditches. Sometimes one windmill was not enough to pump the water to the higher surrounding ditches so a series of millis were used. All these projects were financed by the many rich trader and businessmen. When a polder was dried farmers could rent a piece of land. In 1900 more than 5500 km2 was reclaimed land.