About six kilometers northeast of Maastricht is a former underground quarry for extraction of chalk and limestone. In the spring of 1944, the German air force (Luftwaffe) began to build a workshop in the quarry to repair engines for bombers. The extensive tunnel system and the fact that the mountain itself constituted a natural protection against allied airstrikes made the site suitable for the purpose. The tunnels were expanded and reinforced with concrete floors and roof pillars. Some buildings were also built outside the tunnels. In total, the plant consisted of about 13 000 square meters. However, the workshop was never expanded to its full potential when American forces took over the area in mid-September 1944.

Current status: Demolished with information board (2022).

Location: 50°52' 14.69" N 05°46' 36.89" E

Get there: Car.

My comment:

For safety reasons, the mountain areas have long been closed and sealed. Only small openings allowing bats and other small birds/animals to enter and exit. Along the mountain there are ruins left of what were probably storage buildings.

Follow up in books: Ulrich, Herbert: Hitler’s Foreign Workers: Enforced labor in Germany under the Third reich (2006).