The Dutch unique improvement.
The Polder is the Dutch unique improvement. It can only be built on marshes and flood plains. It provides 3 and additional and after economics is researched.
A polder is a low-lying tract enclosed by dikes with the water drained. In general, polder is land reclaimed from a lake or seabed, from flood plains, or from marshes. In time, the drained land subsides so that all polders are eventually below the surrounding water level. Because of this, water seeps into the drained area and must be pumped out or otherwise drained. The dikes are usually made of readily available materials, earth or sand; in modern times these barriers might be covered or completely composed of concrete. The drained land is extremely fertile and makes excellent pasturage or cropland.
The first polders of reclaimed land were constructed in the 11th Century AD, although building embankments as barriers to water date back to the Romans. The Dutch have a long history of reclaiming marshes and fen-land, and even the seabed, and possess half of the polder acreage in Europe. Although there are polders in other countries of Europe, and examples can be found in Asia and North America, the Netherlands have some 3,000 polders accounting for about 27% of the country's land. Amsterdam itself is built largely upon polder. As the Dutch homily states, "God made the world, but the Dutch made the Netherlands."