The city of Mendig in Germany has northwest of the railroad station an area of about 0.3km² which is riddled with old cellars. This underground labyrinth was created by the quarring or mining of basalt. The basalt was used for millstones, which were exported and became the basis of the wealth of the city. They were sold all over Europe.
Later, after the mining had ended the constant temperature of the underground chambers was discovered. There were cellars with different temperatures, between 7°C and 2°C. The 7°C are the natural temperature of the rock, lower temperatures are caused by the cold trap effect. Cold air is flowing into the cellars through openings in the ceiling during winter, but in summer the cold air, which is heavier, is trapped inside the cellars. This is the ideal situation for the storage of beer, and so the cellars were used as beer cellars starting in the early 19th century. In 1900 28 breweries existed in Mendig and all of them stored their beer in the cellars.
The city of Mendig is a hotspot of the Geopark Vulkaneifel. Three sights have been united and called „Vulkan Dome“: a modern museum, the beer cellars, and the nearby quarry named Museumslay. The museum explains the geological background of volcanism in general and the somewhat special Eifel volcanism. There are numerous interactive displays and a simulation of a volcanic eruption. The Museumslay (museum quarry) is a sort of open air museum showing the machinery and techniques used to quarry the basalt. Lay is a local dialect word used for rock, but also means quarry.
