The Fens used to be a huge area of bogland where rivers converged on The Wash between the main expanse of England and Norfolk protruding up parallel forming the East coast. Now they are drained, forming a huge (by British/European standards - not by the scale of the USA) of arable land.
The draining of the Fens started in the 1600's, but they did not take their current form until the industrial age, where with steam pumps the Fens could be kept drained regardless of natural conditions. The steam pumps were replaced by diesel-engined pumps as soon as possible (in the 1920's/1930's) as there is no coal in East Anglia. Now electric pumps do the day-to-day pumping, with diesel pumps on stand-by should the electricity supply ever be disrupted. The Fens are still settling, albeit much more slowly now, so smaller Fenland roads tend to be bumpy and undulating. So, unique in Britain, the Fens are an expanse of absolutely flat arable land with straight borders and few roads.
Other sites on the Fens ->
The odd feature I have tried to show here is that the rivers in the Fens are raised above the level of the land. The rivers flow by gravity to the sluice at Denver, near Downham Market, which opens at low tide to let the fresh water flow out then closes to keep back the salt water. But that puts the rivers in raised embankments, to which the rainfall not used by the crops must be pumped.
In this picture, the sun is setting behind me as I stand on the river bank and my shadow is cast on the land below.
Nearby, I managed this picture where in a single camera shot you can see the river and see the land which is at a lower level. The brick building is a diesel pumping station. The engine looks "old" (1930's?), but there is an up-to-date telephone directory on the engine-person's desk, so maybe it remains on stand-by (there are electric pumps just beyond the engine-house).
Taken earlier, at a nearby location on the other side of the river. A typical sight in the Fens, if you know what to look out for. The drainage channels for a fen lead to a pumping station set beside a river. The river is in the high bank seen crossing the view behind the pumping station. The modern electic pumps are the two blue things to the left of the engine house, and again one can assume the original diesel pump will remain as stand-bys.