Not far from the fish pond on the Forstbach, a tunnel leads deep into the mountain. The water pipes indicate that a spring bubbling in the mountain is being tapped, the water of which also runs in a channel through the shaft.
You can follow the shaft a good 85 meters into the mountain. Torch is required as it gets pitch dark after a few meters. The shaft then goes further, but only with high rubber boots and sticks through deep mud.