 Since the Swabian Alb is largely shaped of porous limestone, and since the Rhine's level is much lower than the Danube's, today subsurface rivers carry much water from the Danube to the Rhine.On many days in the summer, when the Danube carries little water, it completely oozes away noisily into these underground channels at two locations in the Swabian Alb, which are referred to as the Donauversickerung (Danube Sink).Most of this water resurfaces only south at the Aachtopf, Germany's wellspring with the highest flow, an average of , north of Lake Constance—thus feeding the Rhine.