Great underground cities whose only aboveground sign is some well-concealed air vents and elevators is something I've often fantasized about, and most of the digging work has already been done, such as by the massive mining operations under New York. Given the width and stability, such areas could
easily be converted into those housing developments and shopping malls that are the blight of the land everywhere else, and the surface above them could be converted into forest and farmland at about the same rate.
I once heard an thoughtless suggestion about putting wood down in those massive coal tunnels so "some" years down the line our descendants would have more coal, and thought myself that perhaps we should use those places for all the tonnes of filing paper the U.S. government produces; it would accomplish the same purpose!
(To whom it may concern:) The reason that spaceflight costs so much
money is because, even if it had perfect efficiency, it is still the process of trying to move a very large amount of mass against a very, very large resistance through a very, very, very large distance. No matter how many new technologies we come up with to accomplish this, it is still using more energy than you could ever output in your lifetime, and within very tight constraints. So, the key is less building great space elevators or automated mining systems, and much more insuring that the political and economical systems are efficient and robust enough to support such projects. Those of you following politics might have noticed is has a natural tendancy to have the absolute minimum of these qualities possible yet still sustain itself.
To sum; space-flight/colonization is inherently expensive and requires capability to handle great expense first. Why can't we get those working right first?