In the Middle Ages a cheap way to get building stones was getting them out of a marl quarry. Marl is a soft type of stone, that can be cut out with a big saw. In one day, a day worker could get a block that weighed as much as one ton ready for shipment. The issue with marl is that it turns green in the sunlight. Because of rising labor costs the mines were closed, also the ones in Valkenburg.
Years later, they found out that during the mining they had created tunnels from the Fluweelengrot all the way to the castle.
During WWII the tunnels turned out very usefull. First they served as hiding place for the people of Valkenburg. After the liberation they served as a field hospital.
These days the Fluweelengrot is open for the public. You can visit the tunnels and combine it with a visit to the Castle.