Sydney has underground shopping in the CBD, although it’s not as extensive as that in Toronto and Montréal. There are also underground train lines.
Vancouver has some old railway tunnels (one or two taken over for Skytrain, part of our transit system), quite a few commercial stretches, and more under Chinatown. I think there’s also a museum and/or theatre that opened recently; I just saw one ad about it a while ago.
Moose Jaw, SK, also had underground rooms and tunnels built by Chinese labourers who were brought in to build the railway. That was co-opted by gangsters during Prohibition because of the convenient rail access to Chicago. (The tour repeats a rumour that Al Capone hid out there for a while when the heat was on.)
Do any Asian cities have large old underground works? All I’ve heard of are western.
There’s a large amount of underground storage in Kansas City due to limestone mining.
“Subtropolis” is reputed to be the largest underground business park, now covering about 5 million sq ft with plans to expand to 50 million. It’s owned by the same family that owns the largest limestone operation in the area, the World’s of Fun amusement park and the KC Cheifs football team - Lamar Hunt.
Nashville has no underground. Bedrock & a high water table.
Spoons: The main underground complex (PATH) in Toronto connects to 5 of 69 subway stations. If you were living next to one of them, you were living somewhere that would now be extremely expensive. =) There are a further two subway stations in a smaller complex uptown, and a few subway stations that connect to underground shopping concourses in business areas. One thing about the PATH is that very few of the stores are open past rush hour. It would be very easy to eat breakfast and lunch every day in the PATH, and to do a substantial amount of shopping (since the Eaton Centre is part of the system), but your choices after 6 or 7 pm would be quite limited. I suspect that most of the other underground shopping complexes are also like this.
When I used to live in New Orleans, I would look at Nashville with pity and think, look at all of this underground space we have and yet they have none. 
(New Orleans is below sea level and you can’t even bury bodies below ground in most places.)
There’s an interresting page in the wikipedia that might be of interrest: Underground city
As for the biggest, I believe it might be the underground city of Beijing, China. Parts of it was recently opened to tourists, but it’s rumoured to be enormous!
Oldest? I’ve heard proposed that the tunnels found under Mohenjo-Daro might not all be for sewers, but possibly some were used for deliveries. Unfortunatelly there’s not much left, but otherwise they’d be at least 4000 years old!
Apart from the Tube, london is pretty much built on tunnels. We have railways (other than the tube), tunnels cut for rivers, foot tunnels, road tunnels, catacombs, cellars etc. and lots and lots of shady military stuff.
These people have produced a (brilliant) map of underground london in the style of the London underground map.
THis is one of the best things I’ve seen on the web in a while. Enjoy.
As well as the ordinary underground railway network in London there is a second set of tunnels just for telephone cables. These tunnels are about 8 feet in diameter and stretch all over central London. There is a walkway in the centre and a series of metal racks each side to support the cables. These were built to avoid the necessity of digging up the road every time some new cables have to be installed.
Another underground complex I can think of is in Arras , Northern France. The tunnels were originally built in medieval times to extract building stone. Then houses were built on top, and the householders used the underground chambers to store cheese and wine. In WW1 these same tunnels were used as underground shelters and to connect to the trenches a few miles away. If you are ever there a visit to the tunnels is well worth it. You can still parts of the WW1 construction.
Lastly there is the vast underground hospital built by the German occupying forces on Jersey in the Channel Islands.
Wild Unsubstantiated Rumor Alert
When I was in the military, I heard it rumored that there is a vast underground network of roads connecting the Pentagon, White House, Andrews AFB, Bolling AFB, and a number of other installations in the area.
Of course, it’s all low-lying swamp land, so I doubt it’s true. But I thought I’d share anyway.
END WURA
One interesting underground complex is the tunnels linking various government buildings in Washington DC. I’ve only been in those that link the Capitol, the House office buildings, and the Library of Congress, but I believe they stretch much further than that, probably as far as the White House. Apart from the pedestrian tunnels, there’s a very small underground railway that links the House and Senate office buildings with the Capitol – it’s not open to the public, only to members of Congress and their staff. At present there’s some massive reconstruction around the Capitol to create a new underground visitors’ centre.
Gibraltar is pretty much hollow too.
It has accommodation, hospitals etc built into it, as well as ammunition storage, pillboxes bunkers etc. It’s a very impressive sight (I saw it as a boy when my dad was staioned there, but it’s not in active use anymore so could well be open to the public)
here’s some pictures (and he won’t be welcome back if the locals find out he thinks Gibraltar is a part of Spain - to say the least it’s a ticklish subject)
Howyadoin,
The main railroad terminal in Yokohama is connected to a massive underground shopping mall. All sorts of twisty passages with markets, restaurants and the like. Got lost one night and it took a while to find my way out!
-Rav
I guess I wasn’t as clear as I could have been. We lived next to a subway station uptown on the Yonge line actually. Dad could get on at the station in our neighbourhood, and ride the subway downtown. At the downtown station where he got off the train, he could connect right into his building or into the PATH network (thanks, BTW, for reminding me of the name of it).
Property values in the old neighbourhood are going up, but it’s not like we were living at Dundas and Yonge, or anything. 
Sorry for the confusion. Hope the clarification helps.
Atlanta has an underground also. I believe it is similar in nature to Seattle’s (i.e. the street level was built up a level and old storefronts were buried) but I don’t think it is nearly as big. IIRC it is a bunch of shops and restaurants now.
There is a fairly extensive underground Pedway in Chicago’s Loop.
Just for comparison, Minneapolis has a second-floor-level Skyway network that bears a striking resemblance to a Human Habitrail linking most of the downtown buildings. Here’s a map of it.
San Francisco’s Moscone Center (conventions and trade shows) spans roughly two square city blocks underground.
Fascinating stuff, y’all. Thanks!
It brings up another question, one that was sitting not quite formed in the back of me 'ead while I was composing the OP. Namely, what is behind our tunneling instinct? Or is it some sort of learned behavior? Because it goes back further in time and is much more widespread than I had thought before yesterday. Thoughts?
Have a look at Mary King’s Close . An underground street from the 17th century. This isn’t an example of a street that was dug underground, it simply disappeared beneath the city as it grew. It was then pretty much forgotten about for the next 350 years.
Some claim that it was deliberately sealed off in 1645 because the occupants of the street had the plague, making it an enormous tomb, but that’s fanciful conjecture played up for the tourist’s benefit. (Oooh! Scary ghosts!) But it is a well preserved example of early subterranean street life.
Houston’s downtown has a tunnel system nearly six miles all together. It connects many of the major skyscrapers. There are tons of shops and restaurants down there and it’s very easy to get lost (speaking from personal experience).
There’s the skating venue at Lillehammer that they built inside a mountain. And let’s not forget the Disney under-parks.
And then there’s these guys.