Like Paris, New York, London, etc… Doesn’t have to be mass transit related. Could be military, or religious, or other?
Typically, how much volume are talking? What’s the biggest? Oldest? Deepest?
Any shared by multiple cities?
For some reason, after rereading my OP, I have Pop Muzik running thru my head.
I know that Montreal has a huge one now.
http://www.tourisme-montreal.org/B2C_Target/News/EN/HTML/566_EN.asp?cookie_test=1
Montreal was going to be my first vote, too. The last time i was there, it was about 20 degrees below zero (celcius), and the underground was about all that prevented me having my face frozen off.
Great city, BTW.
You’re going to need to elaborate on what ‘complexes’ are.
Cold war shelters? WW2 shelters? Other unspecified or unidentified excavations? How about WW2 shelters that were built but never used? Or Gibralter, where a whole hospital was built inside the rock?
Another underground, although not an especially large one, is in Seattle. It consists of the old sidewalks and storefronts of the city, which have sunk and been built on top of. The tour, which i took on my last visit, is very interesting.
Any of those will do! 
(see line 2 of OP)
How about the catacombs of Rome? I’m voting for “oldest” here…
Well, going by line 2 …volume-wise, modern excavations are likely to outweigh anything else. But they don’t necessarily specificy cities (I’m thinking of the Channel Tunnel as an obvious example).
As for old, you can start with the Roman catacombs, and work back from there.
From what you said in your last post, I’d say just about every city in the world has something that will fit your description. Care to be a little more specific?
Line 2 if you count title line. Line 1 otherwise. [winky winky]
I wondered about Rome. Were there any other from that time period? Were they the first or merely the oldest still around?
Auckland certainly doesn’t have a large underground complex. We have no underground transport system (Just in case any aucklanders see this I don’t count the waste-of-money that is the britomart).
We do have North Head though http://www.doc.govt.nz/Conservation/Historic/Auckland-Historic-Areas/North-Head-Historic-Reserve/index.asp. North Head gives awesome views of Auckland and has lots of dark tunnels to investigate.
The best part of it is having a giggle at the money spent building tunnels and guns emplacements to protect NZ from the russians in the late 1800’s! Yep those pesky Russians were about to invade anyday now…what? they hadn’t heard of NZ and we were far too far away to attack? Nonsense! We needed those tunnels!
There are numerous urban legends on the extent of the tunnels and what is still in them (they were actively used during WW1 and WW2).
It is a groovy place to spend a day though.
Okay, sheesh. NOT sewers, not building basements. But a complex, meant for humans to either live, work, or travel in. Or congregate for some other purpose. One fall out shelter does not a complex make, imho. Several, linked together would.
Large military establishments might fit, but we might not have that info available. Large shopping centers like what used to be (maybe still is) in Houston would fit, as would the Seattle stuff already mentioned.
Does that narrow it down enough for a meaningful answer?
Toronto’s underground walkways (all lined with shops and restaurants) are quite extensive.
When I lived in Toronto when I was younger, we lived next door to a subway station. Even in the depths of winter, my father didn’t wear an overcoat to the office. Once he was inside the subway, he didn’t need to go outside until he was making the short walk from the station to our home after work. But he could connect from his downtown subway station right into his office building–and buy pretty much anything and get a meal in the underground network.
Here in Calgary, everything is aboveground. A network of catwalks connects across streets in the downtown area. Nice change from Toronto; at least here, we can see the daylight in the winter.
Kiwi, thanks. That kind of stuff is very interesting. As are the other actual answers so far.
[sits back and waits for more…]
Chicago has, besides it subways, an old tunnel complex that used to deliver inventory and coal, IIRC, into Loop stores. Some years back a construction accident knocked out a barrier wall between the tunnels and the … er, Chicago?.. Illinois? … riiver. Most of the big downtown stores ended up with flooded basesments. The losses of inventory and sales (quite a number of businesses including Marshall Fields were closed for weeks) ran into the tens of millions.
Sunk? No, the city burned and while rebuilding they realized that the major sewer/utility problems the old city had could be alleviated by building the “ground floor” up a story, and building street levels that high as well.
Take the tour again 
I’m glad you appreciated it. It is my favourite place in Auckland. I can’t wait till this one is eventually opened. http://astronomy.swin.edu.au/~pbourke/albert/press.html
It is fascinating to think how much is under the city.
Yeah, whatever. It was over ten years ago. I was conflating the fact that the early city had been largely built on stilts to avoid flooding, and also that the new higher city level was initiated to avoid such problems, with the fact that there was often sinking and collapsing of roadways in the old city.
How about the Havre Underground?
This Underground was always below street level. It was built when the Chinese laborors working on the railroads needed someplace to go where they wouldn’t be harassed by the less enlightened of Havre’s 19th century population, and they brought their businesses with them.
A lot of the Underground is inaccessable now, and some of it was the basements of existant buildings, but it was, in a very real sense, a city-beneath-a-city where people lived and worked. And, yes, there are guided tours.