A couple miles before Tuttlingen, the Danube disappears and seeps away in the river bed into the karst cave system below. In some years you can hike to Tuttlingen with dry feet. The still young Danube is suddenly gone.
Part of the water is lost in the subterranean cave system of the Karst and flows towards Lake Constance, the other part reappears a few hundred meters further and flows towards Tuttlingen.
The city of Tuttlingen is known for its instrument making.