This replica of Roman catacombs was installed in an old limestone mine by architect Pierre Cuypers of Roermond and Rijksmuseum fame. Visits are only possible as part of a guided tour – it's in Dutch but has highlights in English.
Larger bags must be left behind to prevent fresco damage, but there is a baggage room. Extra departures are available on request for groups (€25 plus €9.25 per person).
To get to the entrance, walk down Plenkerstraat, passing the old brewery which faces open-excavated remnants of a 5000-year-old flint mine complete with explanatory panels.