Sources and image credits
In Valkenburg, Christmas City begins underground
Source
Image credits
A marl passage in the Gemeentegrot in Valkenburg.
Photo: Romaine / Wikimedia Commons · CC0 · Image source
.jpg){kind=link}
The entrance to the Gemeentegrot in Valkenburg.
Photo: P.J.L. Laurens / Wikimedia Commons · CC BY 3.0 · Image source
