The Akashi Kaikyō Bridge is alleged to have a main span of 1991 meters. If you don’t mind pillars and cables sticking out of the ground, you could build a great many of them, side-by-side and cover the roadway with dirt. This would get you an area 1991 meters wide and indefinitely long.
One cost around 500 billion yen, but you could probably get a volume discount. 
Oh, and Bazlord, welcome to the board. We always appreciate well written informative posts, even five years late.
Does he know that zombies often live in underground spaces?
Disneyworld follows the OP, but remember that due to the high water table in the area, the center of Disneyworld is 2 stories high. The tunnels aren’t actually underground, the center 9 acres was built above ground then covered with fill.
The Wieliczka salt mine in Kraków province, which I read of recently in the Strand Magazine, when it was the Hapsburg Royal Salt Mine, has an underground lake with boating facilities, a cathedral and three chapels all made of salt; is 1,073 feet deep and 178 miles long.
Admittedly it is not one single chamber, but it must include some fairly big spaces.
It ran from the 13th century to 2007.
What about underground natural gas reserves – is that all open space filled with gas? The Haynesville shale, under coastal Louisiana, contains 250-trillion cubic feet of gas, which by my calculation is roughly 1,600 cubic miles, some 2-3 miles below the surface. The would be equal to a room with a 1,000 foot ceiling, the size of New Jersey.
Even if it is not conveniently shaped like a room, it would eliminate the troublesome necessity to remove solid material to the surface, it could all just be rearranged underground.
I believe it is porous rock, not actual cavities.
I presume, then, that porous rock can be pulverized and compressed, to leave large cavities with more compacted rock left behind? Kind of like putting a hot poker through styrofoam, to compact porous material.
Did anyone mention Subtropolis yet? 55 million square feet of man-made space with about seven miles of internal roads used for cool, dry storage. And data.
Closest that I have seen: The Tokyo Flood Tunnels:
(scroll down for the view of the huge chambers with Moria like columns.)
Not sure how effective for a zombie apocalypse. (I noticed now that this is a zombie thread)
This was in the 70s but the biggest underground excavation meant for storage are those crude oil storage tanks. As additional doper knowledge, the longest echo was generated in one of them. Now we've heard it all: Acoustic scientists shatter the world record for longest ever echo | The Independent | The Independent
Engineering-wise, a cavernous storage space is different from several ramifying “human-sized” tunnels, even those with multiple levels. The latter is much easier to excavate and maintain. Mines do it all the time.
Regarding natural caves, I at least know that limestone cave networks are not enduring, even within the span of a lifetime. Limestone caves collapse regularly.
On a different note, the largest above ground structure is the Tropical Island Resort, a fake tropical paradise built inside a giant former aircraft hanger in Germany. It has a volume of 5.5 million cubic meters, and presumably that could be increased substantially if the ground below it was excavated. The size of the oil storage container mentioned below is at most 18000 cubic meters, so it’s not even on the same level–though I imagine you could just extend the length indefinitely.
You might want to check out the Dotiki coal mine in Madison County, Kentucky. The mine underlies the entire county three levels deep and they may, by this time, be working on the fourth layer. Longwall miners using room and pillar techniques + lots of time = vast underground areas.
Not only is the gas in porous rock, but it’s under pressure, and would occupy far less volume than at STP.
For a spherical cavity in an infinite homogenous medium, the tangential stresses at the cavity boundary are twice the geostatic stresses (http://www.usucger.org/PandD/Chapter%2011.pdf) . Taking the compressive strength of rock as 30 ksi and the density as 150 pcf, you could have a sphere with a diameter of 14,400 ft that is only supported by the rock mass.
Now, that elastic solution isn’t valid due to the ground surface being close, there’s no factor of safety, and I’m assuming there are no defects or stress concentrations anywhere. Should give some sort of order of magnitude idea though.
That’s something. I started the virtual tour only to notice that n the second or third “temple” they depict naked men while hiding or even simply not including the genitals. I’m amazed to find prudishness in such a place.
Thats not true. Caves almost by definition form over geologic time. The erosional processes that create the voids occur over LONG periods of time. Any big cave that isn’t a buried pile of rubble has been around a long damn time by human standards. And the buried one even longer, its just not big anymore.
The former Homestake Gold Mine/current Sanford Underground Research Facility in the Black Hills of South Dakota is 8,000 feet deep. I couldn’t find anything about how wide/long it is though.
http://www.theguardian.com/travel/gallery/2014/jan/14/take-a-look-inside-the-worlds-largest-cave
Son Doong cave in Vietnam, recently featured in National Geographic, is said to be the world’s largest. At least 5x larger than Malaysia’s Deer Cave.
http://mysteriousuniverse.org/2014/09/the-lost-forest-world-of-the-worlds-largest-cave/
how many times have you heard on the news that a giant, ungodly deep hole appeared at a street or one’s backyard? A sinkhole is the roof of a cave that has collapsed.
And how many voids that have yet to collapse do you think there are?