The Upper Rhine lowlands are low-lying plains through which the Rhine and Main rivers flow. Contrary to popular belief, however, these are not valleys, they were not formed by deepening of the rivers. The landscape is part of a rift valley structure that stretches from the Rhone Valley to the Leinegraben near Göttingen across Central Europe. At this point, Europe is split in two by the forces of the Upper Mantle. Such rifts are eventually flooded by the sea and form an ocean like the Atlantic, with a mid-ocean ridge. The subsidence of 4 km in the graben is the result of this divergent movement. However, the rift structure developed very slowly and stalled several times.
The Upper Rhine and Main Graben have fault zones at the sides, while in the interior they are built up of horizontally layered sedimentary rocks of the Mesozoic. The low-lying plains are fertile and used for vegetable cultivation. Due to the low location, the groundwater level is high, there is no underground drainage and thus no karstification and no caves. There are only a few mines in the plains; oil is extracted, as are salt and potash. The fault zones at the edge are interesting for mining, but are usually considered part of the surrounding low mountain ranges.
Thus, there are mostly cellars and bunkers in this area.