BLDGBlog proposes Dark City with added spelunking:
It would be interesting to live in a city, at least for a few weeks, ruled by an insane urban zoning board who require all new buildings—both residential and commercial—to include elaborate artificial caves. Not elevator shafts or emergency fire exits or public playgrounds: huge fake caves torquing around and coiling through the metropolis. Caves that can be joined across property lines; caves that snake underneath and around buildings; caves that arch across corporate business lobbies in fern-like sprays of connected chambers. Plug-in caves that tour the city in the back of delivery trucks, waiting to be bolted onto existing networks elsewhere