Caves of Maastricht
The Caves of Maastricht, also known as the caves of Mount Saint Peter or the Maastricht Underground, are a collection of limestone quarries in Maastricht, the Netherlands, the origins of which date back to the 13th century. They were originally dug to mine chalk. In total, a network of 20,000 tunnels were cut into the stone, of which 8,000 are still intact. The caves were eventually no longer used for mining, as the value of lime dropped. Over some hundreds of years, art was drawn on the cave walls, providing an attraction to tourists who can tour  some of the cave sections. During World War II, the cave complex was used to store many valuable paintings and armaments that could be used against the Germans, including a proposed force of 30 tanks that would sortie against the Germans that was timed to attack at the same point of liberation. In addition to the military, over 6,000 citizens took shelter in the caves during World War II. After the war, there were plans to utilise the underground complex as a bomb shelter. Among the paintings hidden in the caves during WWII was The Night Watch by Rembrandt van  Rijn. The canvas which measures 363 × 437 cm. was detached from its frame and rolled into a large cylinder. A large portion of the former tunnel system no longer exists as the encompassing limestone was quarried in an open-cast mine operated by ENCI. Quarrying was ceased in 2018.