Zeller See (Lake Constance) in the context of "Hohentwiel"

Hohentwiel (German pronunciation: [ˈhoːənˌtviːl] ) is a mountain of volcanic origin in the Hegau region of Baden-Württemberg in southern Germany. It is the Hausberg of the industrial city of Singen, located west of it and ca 10 km (6.2 mi) west of Zeller See (Lake Constance). The ruins of the medieval Hohentwiel Castle rest on top of it.

Hohentwiel was an active volcano about 7–8 million years ago during the Miocene epoch, along with several other volcanoes in the Hegau region. It mostly consists of phonolite, which represents the former volcanic pipe. The surrounding softer rocks have been eroded over time by ice age glaciers during the Riss glaciation, creating an inverted relief and giving the mountain its present-day shape.