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
Diane Valkenburg was born on 1984-08-30.
Valkenburg railway station was created in 1853.
Mac Van Valkenburg was born in 1921.
Mac Van Valkenburg died in 1997.
Eric Van Valkenburg is 6' 3".
Robert B. Van Valkenburg died in 1888.