Fens, the, district, E England, a flat lowland, W and S of The Wash. Extending c.70 mi (110 km) from north to south and c.35 mi (60 km) from east to west, it is traversed by numerous streams. The area was originally the largest swampland in England, formed by the silting up of a bay of the North Sea. The higher places were sites of Roman stations. The Romans attempted drainage and built a few roads across the Fens; however, the area had become marshy by Anglo-Saxon times, either from natural causes or from allowing Roman work to decay. The first effective drainage systems were developed in the 17th cent. by Cornelius Vermuyden, a Dutch engineer. Drainage and construction of dikes and channels in the various sections or "levels" continued through the 19th cent., but problems of land sinkage, water accumulation, and periodic flooding existed throughout the period. As a result of flooding in the 20th cent., a drainage-improvement project (completed in the mid-1960s) was undertaken. The district is largely under intensive cultivation. Agriculture is plentiful on the fertile alluvial soils, with vegetables, fruit, and wheat being the principal crops. Wildlife sanctuaries have been preserved. The district is also called Fenland.