Here's a smattering of quickly scanned links from duckduckgo (I have no idea which are more accurate than the others etc. These are just the ones I'd read first. It's all rather 'out there,' to me--yet one wonders.):
Proof Massive Underground Military Bases and Complex Tunnels Are Real (video)
http://www.thesleuthjournal.com/proof-massive-underground-military-bases-complex-tunnels-real/
Tunnels under the USA and Deep Underground Military Bases (DUMBS)
ttp://www.heartcom.org/200under.htm
Deep Underground Military Bases In America
http://nstarzone.com/CODERED.html
Secret Ancient Subterranean Tunnels And Caverns Across America: Who Or What Were Our Ancestors Hiding From?
http://www.ancientpages.com/2015/10/04/secret-ancient-subterranean-tunnels-and-caverns-across-america-who-or-what-were-our-ancestors-hiding-from/
Complete List of Military Underground Bases in USA
http://www.thetruthdenied.com/news/2012/12/14/complete-list-of-military-underground-bases-in-usa/
Richard Sauder Underground Bases and Tunnels--The Best Documentary Ever
https://www.youtube.com/watch?v=hQc8OZ1mbV4
Underground Bases and Tunnels
http://projectcamelot.org/underground_bases.html
U.S. Gov Underground Facilitis and Tunnels Deep Underground Military Base(s)
https://www.youtube.com/watch?v=elsEWGNnCng
That's enough for me to bother with at this point. It's not a favorite topic. I don't know where the boundaries are between real, likely, conceivable & ridiculous. And, I've never cared enough to try & pin such boundaries down--even if that were possible for a layman to do.
Maybe it has occupied some sort of Sci-Fi part of my brain and interests. I don't know. It's one of those mild interests several layers out from my primary interests.
Nevertheless, Thanks for the honor of your question. I'd be curious what you thought of any of those links you followed-up on. I haven't read on the topic in quite a number of years.
Preview is not showing the links at all. I copied the html sandbox instructions to the letter but I may not have been holding my mouth right. So I may try sending them by freepmail. or, I may just copy them again without the html tags trying to make them clickable. I'm obviously not succeeding, if the preview is any clue.
Thanks for that response. Now I got a few rabbit holes to dig in. Let you know what I find.