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Tract of land that is situated below sea level. Protected from flooding by dikes and sea walls. Groundwater levels tend to rise up to sea level, therefore a polder needs continuous pumping to keep the ground seepage from flooding the land. Originally this was done in The Netherlands with windmills, using the wind's force to drive a pump that moved the water back outside the dike. A polder's pumping installation is called a gemaal.

After the last severe break in sea walls and dikes in 1953, a prestigious project was conceived, called Delta Werken. This project was meant to improve the safety of the lower areas of The Netherlands from severe storms and flooding. Among other things, coastal dunes along the entire country were raised by as much as 5 meters and several large scale construction projects were undertaken. The most sophisticated and famous of these is the Oosterscheldedam, which is a dam that can be opened and closed at will, thereby both keeping the violence of the sea out, but preserving the saltwater climate in the river delta for wildlife and the fishing industry.