[TEXOKIE]

Tunnels....

Weren’t there some associated somehow with Denver Airport?

Also
http://projectcamelot.org/underground_bases.html

https://www.thrillist.com/travel/nation/14-american-cities-with-crazy-underground-tunnel-systems-chicago-boston-new-york

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

[generally]

Excellent sleuthing, TEXOKIE!