The country falls into three natural topographical divisions: the dunes, the lowlands or "polders" (low-lying land reclaimed from the sea and from lakes and protected by dikes), and the higher eastern section of the country. About 27% of the land lies below sea level. A long range of sand dunes on the western coast protects the low alluvial land to the east from the high tides of the North Sea, and farther east and southeast are found diluvial sand and gravel soil. The highest point of land, the Vaalserberg, is situated in the extreme south and is 321 m (1,053 ft) above sea level; the lowest point, 7 m (23 ft) below sea level, is Prins Alexanderpolder, an area of reclaimed land situated northeast of Rotterdam. The most extensive polder is that of East Flevoland in the province of Flevoland; it has an area of nearly 55,000 ha (136,000 acres). Many dikes have been constructed along the lower Rhine and Meuse (Maas) rivers, as well as on a portion of the North Sea coast and along nearly the whole of the coast of the former Zuider Zee (formally called the Ijsselmeer since its enclosure by a dike in 1932). There are many canals in the country, most of which have numerous locks.