dutch valley with the limestone cave duivelsgrot in mount saint peter

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