Close to the village of Hundelshausen belonging to Witzenhausen, Germany a company gathers gypsum. Already in 1940 they gave up an area and the open pit, surrounded by high rocks, filled with ground water and rain. Today it is a lake with 5000 cubic meters of water that you can use for swimming. There are also platforms on the water, a separated non-swimmer area and a wooden base.
The lake is unsupervised and swimming there is on your own risk. You can take your dog with you, there are some benches to sit down and there is also a small cabin if you want to change cloths. Some parking lots are available not far away. Around the lake there is a very narrow path you can walk but I found it in some sections much too dangerous. If you want to reach the rear end of the lake there is also a path leading up the hill close to the entrance.