Maastricht Underground runs spooky, amusing and fascinating tours into sections of the vast tunnel network beneath St-Pietersberg massif. Departures run on a constantly shifting schedule, with up to five tours daily in summer, very few off season – check the website.
The more popular North Cave tours focus on Napoleonic history and start from the same ticket office as for Fort St Pieter. If you get a small group, ask politely and the guide might be kind enough to extend the tour to the fascinating Nazi art-vaults. The bigger Zonneberg (southern) Caves, which sheltered thousands during WWII bombardments, have far fewer visiting slots but are fascinating for their murals, centuries of mining history and escape stories. Zonneberg Cave tours start from behind the Buitengoed Slavante cafe, a 1.5km walk further south (much further by road).