|Laacher See or Laach Lake (in English) is a caldera lake in Rhineland-Palatinate, Germany, situated close to the cities of Koblenz, Mayen (11 km), and Andernach (14 km). Just like the nearby 'Wehrer Kessel' volcano and Rieden volcano, the Laacher See volcano forms a volcanic caldera in the Eifel mountain range. It is part of the area called "East Eifel volcanic field". The great Eifel lake takes its name from the Old High German word "lacha" which in turn is derived from Latin "lacus" as English "lake". Thus the name literally means "Lake of the Lake" similar to the "Loch Lochy" in the Scottish Highlands with the same meaning. The lake lies 259 m above sea level, is 8 km in circumference, and surrounded by a ring of high hills. The water is blue, very cold and bitter to the taste. The lake has no natural outlet and so the water level changes considerably due to evaporation and rainfall conditions. On the western side lies the Benedictine monastery of Maria Laach Abbey (Abbatia Lacensis), founded in 1093 by Henry II of Laach of the House of Luxembourg, first count palatine of the Rhine who has had his castle opposite to the monastery above the eastern lakeside.
The caldera was formed after the Laacher See eruption dated to 12'900 years ago. The remaining crust collapsed into the empty magma chamber below, probably two or three days after the eruption. With an estimated 6 km3 of magma erupted, this eruption was slightly larger than the 1991 eruption of Pinatubo (Philippines). The Laacher eruption concides with the onset of the abrupt Younger Dryas reglaciation, which brought renewed very cold conditions to the northern hemisphere from 12.9 to 11.6 ka. However, any relationship between this climate change and the eruption is speculative; eruptions of the size of the Laacher See eruption usually cause only short-term global cooling. Remains of this eruption can be found all over Europe and the tephra is often used for dating of sediments. A number of unique minerals can be found in the region, and quarries mine the stone as a building material. The Laacher is a potentially active volcano, proven by seismic activities and heavy thermal anomalies under the lake. Carbon dioxide gas from magma still bubbles up at the southeastern shore, and scientists believe that a new eruption could happen at any time, which, today, would be a disaster beyond all description.