interior walls of the limestone cave duivelsgrot in sint-pietersberg

autumn caestert cave cavern caving creepy day devils discovery duivelsgrot earth entrance environment erosion europe explore formation geologic geology grotto ground hill hole holland interior landscape limburg limestone maastricht mountain mysterious nature nederland netherlands nobody opening part plateau quarry reserve rock sand scenery scenic sintpietersberg slope south speleology stone surface texture trail travel tunnel underground underworld velvet view vineyard walls wijngaardgroeve winter