Kasteelruïne & Fluweelengrot in Valkenburg aan de Geul

You're standing at the entrance to the Fluweelengrot, the Velvet Cave, on the Daalhemerweg in Valkenburg. From out here there isn't much to see, but beneath your feet runs a maze of more than five kilometres of tunnels. People made them, not nature. From around the year 1050, block cutters carved marlstone out of this hill, mostly to build Valkenburg Castle. Most of the passages were dug between the fourteenth and sixteenth centuries.