The Ir. D.F. Woudagemaal is a working steam pumping station from 1920, named for the engineer who designed it, still operational, and on the UNESCO World Heritage list. It pumps surplus water out of Friesland and into the IJsselmeer during exceptionally wet periods — typically only a few days a year, but the boilers and pumps are kept ready to run on a few hours' notice.
The visitor centre next door explains how the Dutch keep their feet dry, with hands-on exhibits, an underwater windowed channel where you can see fish, and a model showing the polder system in cross-section. The pumping station itself is open for self-guided visits in summer and on operating days, when the boilers are lit and the air vibrates with the rumble of the pistons. It is a ten-minute drive west of Lemmer; budget at least two hours including the visitor centre.