Yeah had heard that a large portion was below sea level but it just didn't make that much sense to me. Your country is fairly flat though isn't it?
Those 20% of land below sea level were created in an inland sea.
Creating land from sea
At the outset of the 20th century, the present province of Flevoland was just a patch of water in the Zuiderzee. Plans to reclaim the land under the inland sea were proposed as early as 1667, but were technologically impossible at the time. The idea was revived in 1891 by civil engineer and statesman Cornelis Lely. Despite Lely’s assurances about the feasibility of his plans, he didn’t get the necessary support to execute them. Attitudes changed when the Zuiderzee’s surrounding area flooded in January 1916.
In 1918, the Zuiderzee Act was passed and Lely’s plans to enclose the sea and partially drain it were put into motion. The key element of the Zuiderzee project was the construction of the thirty kilometer long ‘Afsluitdijk’ across the Waddenzee (an arm of the North Sea ). It involved moving millions of tons of earth and rocks and was carried out manually by labourers working from each end of the structure. The Afsluitdijk was finished in 1932, turning the Zuiderzee into a fresh-water lake – the IJsselmeer . The Afsluitdijk enabled the reclamation of three large tracts of land: the Northeast Polder (1942), Eastern Flevoland (1957) and Southern Flevoland (1968; location of the current city of Almere ).
