There are plenty of stories about hundreds of underground cities being build here and there in USA, but I've only seen photos of one under the Denver airport (and even so it's no real proof that these photos are related to that place).
But would it be possible to build such huge infrastructures without having a major impact on the economy? As I know USA has a very big military budget, could it be used to hide those operations, if they exist?
Here are some examples:
While the 1st is self-explanatory, the second reference is just a list of key persons without any specific info. I was using this list to Google info on each person, and that's too big a list to put it here. Basically that's where I got those numbers about cities and average population.