The 19 windmills of Kinderdijk are an internationally known Dutch symbol. The 17 basin-mills were built between 1738 and 1740. Two polder-mills remain from even before then. The windmills operated in a polder-drainage system of the two ancient Water Boards of the Alblasserwaard. Later on modern pumping stations took over the role of the windmills. The diesel-powered JU Smit and the electrical ir. GJ Kok pumping stations still operate in the drainage system. In Kinderdijk, almost a thousand years of the 'struggle against the water' are visible in the polder landscape, with its waterways, dykes, pumping stations, sluices and windmills. The Mill Network at Kinderdijk-Elshout has been a World Heritage site since 1997.