Outside of my house there is what the locals nonchalantly call an old 'mine tap' - the entrance to a former mine. You wouldn't know it, as it is underneath the road and completely covered in tarmac. But it is there all the same, the mine itself having apparently been decommissioned in 1935. This worried us when we were first thinking of buying the place. Then we learned that the entire area is riddled with these old mines and, as the estate agent put it, if it was a problem for us it would be a problem for the approximately 10,000 other people who live around us too. I'm not sure why that reassured us, but it did.
In any event, it conjured up an image of an entire hillside (the town in which I live spills down the whole side of a steep escarpment) burrowed through with tunnels, the existence of which is at best at the fringes of the awareness of the locals, and which is accessible perhaps to a small circle of secret urban spelunkers.
I was going to say that this screamed D&D to me, but that wouldn't be strictly accurate. It screamed 'modern dungeoneering', but D&D is a singularly bad system for modern settings. This is something I am sure have written about before, though I can't quite remember where, but the abstraction of D&D combat and the concept of hit points disintegrates once guns are involved, and the 'feel' in play of levels, XP and so on is all wrong for the real world; anyone who doubts this need only try to run d20 Modern, the game that time forgot.
No: the closer an RPG setting is to our own experience, the more crunchy I think it needs to be. We can suspend disbelief about all manner of things when dragons and magic missiles are there to be imagined. When the game is set in our reality, in contemporary society, we are keenly aware of what is and is not possible. We want a system that reflects that - at least if action and derring-do are going to be involved. Our thoughts turn to GURPS, to CP:2020, to the Hero System, and so on. We want things to feel as though they matter, because to us the real world matters - we experience it not as imagination, but as 'lived experience', and loose approximation won't do.
What lives in the old mines buried beneath a city? Ghosts, giant vermin, cultists, murderers? No doubt. A slumbering god? Quite likely. Demonic entities awakened by the sound of mining, Moria-like? For sure. But what is the PCs' aim? Not amassing gold, perhaps - not literally, anyway - but once one has access to the mines, the geography of the town is entirely rearranged; one is not bound by roads, walls, pavements, fences, but by the tunnels themselves, and the tunnels might go anywhere. The ability to appear and disappear as if from nowhere is surely a burglar's dream - not to mention an assassin's, paparazzo's, or private investigator's.