Laacher See

Laacher See, also known as Lake Laach or Laach Lake, is a volcanic caldera lake with a diameter of 2 km in Rhineland-Palatinate, Germany, about 24 km northwest of Koblenz and 37 km south of Bonn, and is closest to the town of Andernach situated 8 km to the east on the river Rhine. It is in the Eifel mountain range, and is part of the East Eifel volcanic field within the larger Volcanic Eifel. The lake was formed by a Plinian eruption approximately 13,000 years BP with a Volcanic Explosivity Index of 6, on the same scale as the Pinatubo eruption of 1991.

Source: Wikipedia