Laacher See or Laach Lake (in English) is a crater lake or more exactly a caldera lake in the Rhineland-Palatinate, Germany, situated close to the cities of Koblenz, Mayen (11 km), and Andernach (14 km). It fills a volcanic caldera in the Eifel mountain range, the only caldera in Central Europe. It is part of the area of the "east Eifel volcanic field".

The caldera of Laacher See was formed after the Laacher volcano erupted, between 12,900 and 11,200 years ago. The remaining crust collapsed into the empty magma chamber below, only two or three days after the eruption. With an estimated Volcanic Explosivity Index value of 6, this eruption was 250 times larger than the eruption of Mount St. Helens in 1980. Remains of this eruption can be found all over Europe and is often used for dating of sediments. A number of unique minerals, like hauyn can be found in the region, and quaries to mine the stone as a building material.
 
The Laacher is still considered to be an active volcano, proven by seismic activities and heavy thermal anomalies under the lake. Carbon dioxide (CO2) gas from magma still bubbles up at the southeastern shore (mofettes), and scientists believe that a new eruption can happen at any time, which, today, would be a disaster beyond all description.