Turns out you didn't even need those secret tunnels, there were plenty of other holes and passageways that were taller but still pitch black and possibly unsafe. I started climbing up a tower, expecting it to be blocked off eventually, but it just kept going. Stephanie had to get out the light on her phone so that we could continue walking, not knowing where we'd end up. And there were multiple places similar to that, and we were eager to find and explore them all.
Cost: 4 € for adults
Time to Explore: 2-3 hours, longer if you crawl though the tunnels
Opening Hours: 9am-6pm (March to October), 9am-5pm (October to Mid
November), 11am-5pm Weekends (Mid November to
March, weather permitting)
Tips: Bring a flashlight for exploring!