The village of Kinderdijk is a popular tourist destination because of its many what?

Kinderdijk is a village in the Netherlands, belonging to the municipality of Molenwaard, in the province South Holland, about 15 km (9 miles) east of Rotterdam. Kinderdijk is situated in a polder in the Alblasserwaard at the confluence of the Lek and Noord rivers. To drain the polder, a system of 19 windmills was built around 1740. This group of mills is the largest concentration of old windmills in the Netherlands. The windmills of Kinderdijk are one of the best-known Dutch tourist sites. They have been a UNESCO World Heritage Site since 1997.

In Alblasserwaard, problems with water became more and more apparent in the 13th century. Large canals, called "weteringen", were dug to get rid of the excess water in the polders. However, the drained soil continued to subside, while the level of the river rose due to the river's sand deposits. After a few centuries, an additional way to keep the polders dry was required. It was decided to build a series of windmills, with a limited capacity to bridge water level differences, but just able to pump water into a reservoir at an intermediate level between the soil in the polder and the river; the reservoir could be pumped out into the river by other windmills whenever the river level was low enough; the river level has both seasonal and tidal variations. Although some of the windmills are still used, the main water works are provided by two diesel pumping stations near one of the entrances of the windmills site.