If a 'necropolis' is a city of the dead and a 'metropolis' is a main city then what is the term for an underground city?

Answer 1

I know of no such word, but one can be reasonably built using accepted English conventions. This doesn't make it a dictionary-approved term, of course. But, if it follows standards of the language, a case can always be made for it's validity.

Conceivably, an underground city could be a terropolis (or 'terrapolis', 'teropolis', 'terapolis'... choose any spelling that fits your liking, but they would all have the exact same pronunciation). I'm using the prefix terra-, originally from Ancient Greek, which means 'land' or 'earth'. Thus, a 'terropolis' would be a 'city of (in) the Earth'.

Even a better fit might be subterropolis, adding sub- to denote 'beneath'. This term would literally indicate a 'city beneath the land'... a subterranean city.

Of the two possibilities offered, the former is a less bulky word but is comparatively imprecise. The latter is precise, but it's overall construction is somewhat awkward, I think... perhaps unnecessarily so.