Lately we took that trip to Belgium, I have already written about. We also went to the Grottes de Han. Those are - I think - Europes biggest caves.
The guided tour was great and infomative and even got exciting at one point when the lights switched off by accident and we had to make a part of the way in the dark with a single flash light of the guide.
At the exit of the cave they switch the lights off for a moment and let you enjoy the view out the opening.
Here is what that looks like, seen over the river that dominates the cave and periodically floods it every year.