The Fluweelengrot (Velvet Cave) is a network of tunnels beneath Valkenburg, adjacent to the castle cave system. The name comes from the velvety green algae that grow on the walls in wetter sections. The caves were quarried for building material over centuries, and the stone extracted from them — a warm yellow marl — was used to construct much of the town and surrounding villages.
Guided tours are available separately from the castle cave tour and cover different tunnels. The highlight for most visitors is the World War II section, where the story of the evacuated masterpieces is told with photographs and replica materials. The caves are cool and damp throughout; suitable footwear and a light jacket are recommended.