You are looking at a volcano. It is the Kaiserstuhl, on the western side of the Rhine Rift Valley. I was standing on the eastern side, at the base of a ruined castle.
The Germans are firm believers in the idea that if you back off a cliff taking selfies, or wander along the edge and lose your balance, you should know better and it’s your fault. I heartily endorse this philosophy. But back to volcanoes.
The best known volcanic activity in what is now Germany is in the northern part of the Rhine Valley, in the Eifel. But there are other places with evidence of vulcanism, including the Kaiserstuhl. The Rhine remains tectonically active, with hot springs and the occasional earthquake just to keep people wary.
The Laachersee was, and is, a major volcano in northwestern Germany, in the Eifel. It erupted enormous amounts of tephra and ash, enough to ruin a lot of critters days back 12,900 or so years ago. So pretty recently, although not as recently as the Puy de Dome in France. The volcano then collapsed in on itself, forming the lovely lake that you see today, which happens to be bubbling again. Just CO2, nothing really interesting…yet.
There are lovely, perfectly round lakes in the area near the Laacher See, called Maar. The term comes from the Latin “mare” meaning sea, and is a dialect term that has become official geology language. Maars are perfectly round little crater-remains, often with water in them but not always, that are the result of certain kinds of eruptions. The Eifel has the type-sites, and is a neat place to visit if you are into geology.
The Kaiserstuhl is a wee bit older, as in 16 or 19 million years old, and quite dormant. It was probably six thousand feet tall at its greatest, but has eroded since then. The name comes because Otto III held a meeting there. It is part of the Rhine Graben volcanic complex in the southern part of the valley, not far from the French and Swiss borders.
Today, only the Laacher See is considered dormant. The rest are either extinct or very extinct, although there are a few spots on the Czech border that might be questionable. I’d put my money on the western part of Germany if I were betting on vulcanism.
There is a really wonderful book about vulcanism in Germany, but it is entirely in German and assumes that you have some basic geology and geochemistry. Until you read something like that, you have no idea just how much geological excitement took place of the the years in the area that is now Germany. Granted, it wasn’t happening all at once (thanks be!), but northern Europe was a happening place for many thousand years.
The website below has more information about three of the volcanic regions in what is now Germany.