The polder is in fact a basic unit of Dutch territory. About 40 percent of the Netherlands is below the level of the North Sea at high tide — and the whole country is pretty flat and low — the northern European delta.

Basically the Northern European delta, Holland is low land indeed. [This is not to mention that the ground-level in many areas has subsided* in historic times.]

The Dutch (the Frisians, really) originally began to claim land by means of the "terp," a mound of earth built high enough to stay dry at high tide.

The average Dutch polder has a surface area of 5 square kilometers (2 square miles.) The smallest polder has an area of 1.5 hectare (3.7 acres.) The largest extends over a space of 540 square kilometers (208 square miles.)

Then came dikes, which allowed a greater area of dry land for less earth-work. A polder is just what you get when an area is "bedijked." Instead of property raised by addition of sand, a polder is land claimed by exclusion of searise and evacuation of rainwater.

Originally, polders were drained by manually-operated watergates, which residents opened at low tide for the outflow of rainwater accumulation. Later came the watergate that operated as a valve — allowing water to flow out but not in. Naturally, watergates were only useful on lands above low-tide level.

Various inventions based upon various principles, including leverage and the Archimedian screw, allowed humans to enhance their power, and that of their animals, in moving undesired water to the outside of a polder. But it was all still hard work, and the settlement of low-lying terrain was a slow process.

In the 1700's, wind-driven devices enabled the Dutch to claim huge areas of land by pumping water to higher levels. An arrangement of small, low canals leading to larger, higher canals allowed water to then flow seaward by gravity.

Much of modern Holland is low enough to submerge a tall man if the water were still there. The lowest altitude of substantial area — according to the Minister of Transport, Public Works and Water Management — is West of Rotterdam, at 7.64 meters below mean sea level. About 24 feet.