Tunnels....
Weren’t there some associated somehow with Denver Airport?
Also
http://projectcamelot.org/underground_bases.html
https://untappedcities.com/2015/10/05/7-secret-nyc-tunnels-you-probably-havent-heard-of/
https://en.wikipedia.org/wiki/List_of_tunnels_in_the_United_States
https://www.timeout.com/newyork/things-to-do/secret-underground-spaces-around-nyc
Underground military bases
http://nstarzone.com/CODERED.html
http://actionsquad.org/underground.html
[haven’t looked but this one should be fun:]
http://www.reptoids.com/Vault/AllNewArticles/Tunnelsandshuttles.htm
Excellent sleuthing, TEXOKIE!