entrance to a cave in a limestone hill with a closed wrought iron gate

autumn cannerberg cave cavern caving closed cloudy creepy day dry entrance environment erosion europe explore formation gate geologic geology green grotto ground hill hole holland iron landscape leaves limburg limestone loess maastricht marl mount mountain mysterious nature nederland netherlands nobody old opening outdoor plants plateau quarry reserve rock sand scenery scenic slope south speleology stone surface trapping tunnel underground underworld velvet view wall wrought