Not exactly ‘urban’ but we had a great trip to Verdun a few years ago where I went poking around in an underground WW1 fort. Lots of old gun raising and lowering machinery etc – very interesting.
I had to pretend that ‘Danger, defense d’entrée’ meant ‘Welcome, come in and take a look around’. I expect that one can still get into a few of those old places – found a website about a lot of the old forts around Verdun recently – I want to go back and take another look!
I found Verdun to be a very dark place – makes the Somme and Flanders feel quite cheerful.
I’ve spent some time poking around in Welsh mines as well – great fun. Just take a look at the roof of any tunnel that you plan to enter and make a call as to how long it’s going to stay up before you go in! The silica mine at Glyn Neath is good for a bit of snorkelling if you fancy it – gin clear water…