Cannerberg

Like so many hills in this area, the Cannerberg is peppered with underground passages due to centuries of limestone mining.
The quarries in the Cannerberg have a rich war history more than other quarries. This started on a larger scale during the Second World War, when the Germans set up a secret workshop and launch pad for V1 rockets in the Cannerberg.
After the war, part of the quarry became an 'ultra-secret' part of the Netherlands, because a NATO headquarters was established here until 1990. The complex has been governed by the State Secrets Act since 1956.
When tensions rose in 1963 due to the Cold War, the headquarters was given a permanent occupation with about three to four hundred troops present during the day.
Dutch, Belgian, British, German and American soldiers manned it day and night in case war broke out. In 1992, NATO left the quarry.

Nowadays you can take a guided tour, where you dive into the impressive underground structures. For example, the middle corridor was renamed Main Street and the rest of the corridors were given names from the NATO alphabet.
As in many bomb shelters from the Cold War, overpressure ensured that hazardous substances could be kept out in the event of a nuclear, biological or chemical attack.
In addition to four hundred offices, two restaurants and a hair salon, there was even a 3-hole golf course with artificial grass.

Office de tourisme du Sud du Limbourg (Visit Zuid-Limburg)