Maastricht's visitable caves/mines are underappreciated;
I am always surprised to see so few foreigners visiting these.
One used to be able to visit an old mine, being led around by one of the old/last real miners;
I met one such old miner myself about 10 years ago;
Unfortunately they have almost all died out by now (mostly due to silicosis);
their numbers have dwindled, like the veterans at the WW2 remembrance ceremonies.
I'm gonna be hitting europe in july for a few nights (Cologne Germany) and plan on visiting all the surrounding countries. Will probably train to some, plane to others. I've never really traveled by train in my life, other than a few subway rides, and the two times I took the long island railroad.