A quick search reveals they are not natural caves but former underground building-stone quarries that have been used for other purposes since.
They were originally building-stone mines.
Dirk Valkenburg was born in 1675.
Dirk Valkenburg died in 1721.
Oud-Valkenburg's population is 130.
Patti Valkenburg was born in 1958.
Paul I. Van Valkenburg has written: 'The Van Valkenburg family in America' -- subject(s): Genealogy
Eric Van Valkenburg is 6' 3".
Diane Valkenburg was born on 1984-08-30.
Mac Van Valkenburg was born in 1921.
Mac Van Valkenburg died in 1997.
Valkenburg railway station was created in 1853.
Pete Van Valkenburg was born on 1950-05-19.