The polder is a unit of land enclosed by dikes and managed as an independent system.below the local water table.
All of west Netherlands except rivers and a string of dunes is below sea level, and comprises about 3000 polders.
Most of this land, a delta peat bog in early modern times, was above sea level before settlement, and has degraded with exposure to oxygen. Polders, their systems, and a culture evolved as the ground subsided.
The average size of a Dutch polder is 5 square kilometers (2 square miles.) The largest is 540 sq. km, the smallest 0.015.
Source: Landelijke Informatielijn Rijkswaterstaat
There had been polders from centuries earlier along the coastline, bediked areas holding out high tide and releasing excess water at low. But the great majority of polders formed and the technique evolved on inland farms that could no longer be drained to outside water.
During the 17th century, there came projects that really did make land of watery territory. By the time of the Dutch Golden Age, there were powerful standard-model water windmills, and the enormous wealth of the era permitted their deployment in huge numbers.
In 1612 the Beemster, a lake formed by peat harvest and widened by stormflood erosion into a problematic arm of the Zuiderzee, had been bedijked and pumped by the use of windmills.
Following the technical (eventual) success and enormous profitability of the entrepreneurial drainage and division of the Beemster into farmland parcels, the rest of the delta's shallow lakes were dried within the next few decades. Political will was easy because the lakes were a stormflood hazard and an erosion menace and there was plenty of capital for good lakebed farmland. The windmills had created and maintained a couple hundred square kilometers of terrain.
It wasn't until 1811 that anybody would initiate another major *landwinning* project, the Zuidplaspolder in South Holland province. Centuries of peat harvest had led to lake Zuidplas, and it was a menace to bordering properties. Also, the 19th century brought again a high demand for arable property. Zuidplas was drained using windmills, and a few decades later the Netherlands' first large steam pumping station replaced them.
The steam pump-station came into its historical moment in the early 19th century when the technology coincided with sufficient political will to drain the great Haarlemermeer, a menacing body of water in the south of North Holland province. Adjacent municipalities had since the Middle Ages harvested peat from the deepening beds of lakes that had eroded into one lake that ate villages during stormy weather. The Haarlemermeer between 1848 and 1852 was drained using three steam pumping stations.
In 1891 Cornelius Lely drafted a plan for closing the Zuiderzee, a great shallow inland sea extending into the heart of the Netherlands. There was opposition from fishermen amongst others, but it was problematic in certain weather/tide conditions, and after the 1916 stormflood Plan-Lely became reality. A 1918 law decreed the closure and impoldering of the Zuiderzee. The first world war delayed execution of Lely's plan, but also showed the importance of food security and a greater expanse of farmland.
An experimental polder completed in 1927 showed the arability of the dried seabed. In the next few decades Plan-Lely claimed more than 1700 square kilometers by the use of large electric pumping stations. Much of the former Zuiderzee (now the freshwater lake Ijselmeer) is now farmland. Part of that is the twelfth province.