A polder is the unit of land enclosed by dikes and managed as an independent hydrological entity below the local water table.
All of west Netherlands except rivers and a string of dunes is below sea level. But it wasn't, before cultivation. In Roman times (the first recorded,) low Netherlands was raised bog, several meters above. Peat had been accumulating since the end of the most-recent Ice Age mosses and other flora growing in acidic, anaerobic, nutrient-poor stagnated rainwater. Without oxygen, it lay in stasis and piled higher, slowly.
Settlement was late in the region, outside of the sparse and sporadic incidence of high ground. The bogs themselves were poor farmland without extensive work, time, and cooperation, and history didn't encourage their occupation until the early Middle Ages.
Around 1000 CE, the region developed a need for arable property. The Vikings were gone; the plow had improved; and the climate had warmed a little. The introduction of livestock augmented the labor force and fertilized the soil.
The earls of Holland and bishops of Utrecht began granting charters for the settlement of large parcels of wilderness to friends and political associates, who then alloted parcels to individual farmers. The farmers, these colonists, received outright ownership of a strip of land in exchange for easy long-term conditions. (It would take a long time to see harvest.)
By about 1300 the (now-)"Green Heart" of the Netherlands was cultivated. Since then, it has been sinking.
The way that polders evolved is an intricate story involving a complicated hydromechanics at the heart of Dutch history and culture. But the basic outline of that story is that their implementation was adaptive to the subsidence of land soil decomposing on exposure to oxygen.
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 into outside water because the land had subsided.
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 shallow inland sea extending into the heart of the Netherlands. Important historically, it was problematic in certain weather/tide conditions. There was opposition from fishermen amongst others, but after the 1916 stormflood Plan-Lely became reality.
The 1918 (Zuiderzee proclamation) legislated 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 great electric pumping stations.