In the location's words:

The Rurberg natural outdoor pool is located in the Eiserbachsee directly at the Rur dam in the Eifel. The large sunbathing lawn at Eiserbachsee is about an hour's drive from Aachen. There is also a shallow water area for young swimming beginners. For sports enthusiasts, there is a volleyball and soccer pitch. Admission is free, but there is a charge for parking. The cute bistro has ice cream and snacks for your swimming break.