Fun stuff...
DA: Now, you mentioned that underneath this airport it goes down many levels. Does the fact that all these underground levels are there have something to do with why it took so long for this airport to open?
AC: Well, the gentleman that I was dealing with, Phil Schneider, said that during the last year of construction they were connecting the underground airport system to the deep underground base. He told me that there was at least an eight- level deep underground base there, and that there was a 4.5 square mile underground city and an 88.5 square-mile base underneath the airport. It is very unusual that they would allot a 50 square-mile area on the surface at which to locate an airport in the middle of nowhere unless they really planned to use it for something very unusual later.
Denver Airport Underground base and weird murals