I found a "DUMBS" file on my computer from April 2020 containing many links.

http://www.thetruthdenied.com/news/2012/12/14/complete-list-of-military-underground-bases-in-usa/

https://www.militarytimes.com/news/your-army/2019/02/26/the-subterranean-battlefield-warfare-is-going-underground-into-dark-tight-spaces/

https://www.military.com/daily-news/2018/06/24/army-spending-half-billion-train-troops-fight-underground.html

https://sputniknews.com/military/201806251065742558-US-Army-Training-Underground-Combat/

https://www.gizmodo.com.au/2014/02/training-for-underground-warfare-at-a-nuclear-weapons-complex-in-texas/

https://popularmilitary.com/us-army-training-soldiers-fight-subterranean-warzones/

https://www.timesofisrael.com/idf-paratroopers-teach-us-marines-lost-art-of-tunnel-combat/

https://www.defenseone.com/technology/2018/06/underground-may-be-us-militarys-next-warfighting-domain/149296/

https://taskandpurpose.com/news/army-subterranean-warfare-training

https://www.popularmechanics.com/military/weapons/a14456933/the-us-army-is-training-to-explore-fight-in-north-koreas-maze-of-tunnels/

https://www.npr.org/2018/01/09/576542410/as-north-korea-tensions-rise-u-s-army-trains-soldiers-to-fight-in-tunnels

https://mwi.usma.edu/elephant-tunnel-preparing-fight-win-underground/

https://taskandpurpose.com/military-tech/army-robots-subterranean-warfare

http://www.leatherneck.com/forums/showthread.php?60477-Marines-build-warfare-training-city-in-desert

https://mwi.usma.edu/fixing-way-army-trains-urban-warfare/

https://en.wikipedia.org/wiki/Tunnel_warfare

https://smallwarsjournal.com/jrnl/art/swj-book-review-underground-warfare

https://www.desertsun.com/story/news/2015/09/04/marines-train-urban-warfare-simulated-scenarios/71729350/

https://www.lawfareblog.com/fighting-tunnels

https://www.businessinsider.com.au/darpa-underground-spaces-prep-urban-warfare-2019-8?op=1&r=US&IR=T

https://www.businessinsider.com.au/trump-pushes-for-space-force-as-us-army-preps-for-underground-warfare-2018-8/?op=1&r=US&IR=T

https://www.airforcetimes.com/news/your-military/2019/02/26/tunnel-warfare-will-test-the-air-forces-ability-to-spy-and-bomb-a-hidden-enemy/

https://www.airforcetimes.com/news/your-air-force/2019/03/12/the-air-forces-struggle-to-fight-subterranean-warfare/

https://www.nationaldefensemagazine.org/articles/2002/10/31/2002november-navy-sets-up-new-facility-for-tunnelwarfare-training

https://qz.com/1175447/north-korea-the-us-army-is-training-to-fight-in-tunnels/

https://www.ausa.org/articles/facing-our-underground-nightmares-casting-light-subterranean-fight

https://www.newsweek.com/us-war-north-korea-tunnels-775209

https://www.c4isrnet.com/unmanned/2019/06/03/how-the-military-could-find-robots-it-needs-to-win-underground/

https://www.defenseone.com/technology/2017/12/pentagon-launches-new-push-tunnel-warfare-tech/144748/

https://military.wikia.org/wiki/Tunnel_warfare

https://defense-update.com/20140727_subterranean_ops.html

https://www.breitbart.com/middle-east/2018/03/14/marines-train-israeli-military-urban-warfare-drill/

https://www.popularmechanics.com/technology/design/g152/strangest-military-bases-gallery/

https://www.fpri.org/article/2015/04/fighting-under-the-earth-the-history-of-tunneling-in-warfare/

https://www.washingtonpost.com/news/checkpoint/wp/2014/07/21/hamas-tactics-highlight-u-s-militarys-preparation-for-tunnel-warfare/

https://apps.dtic.mil/docs/citations/ADA620416

https://mwi.usma.edu/underground-warfare-wish-list/

https://global.oup.com/academic/product/underground-warfare-9780190457242?cc=au&lang=en&

https://mwi.usma.edu/war-books-preparing-underground-warfare/

https://www.law.columbia.edu/events/underground-warfare

https://callofduty.fandom.com/wiki/Underground

https://roblox.fandom.com/wiki/Community:Stickmasterluke/The_Underground_War

https://youtu.be/ylkEfC-CoyIYouTubeYouTubeYouTube

https://survivalschool.us/survival-info/military-manuals-pdf/

https://www.reddit.com/r/DrainTheSwamp/comments/8v1xnj/underground_warfare_happening/

DUMBs and Billionaire Bunkers: >

https://www.youtube.com/watch?v=cLy6AXjwQK8YouTubeYouTubeYouTube

https://www.youtube.com/watch?v=bAOj7pMXVgI&list=PLdB5_JQvO-7KLUOolEkAjrrhZMJgYWjHt&index=20YouTubeYouTubeYouTube

https://www.youtube.com/watch?v=4LuHqhNEKqkYouTubeYouTubeYouTube

https://www.youtube.com/watch?v=BQtjZFUyvSAYouTubeYouTubeYouTube

https://www.youtube.com/watch?v=L8jU67dOH7kYouTubeYouTubeYouTube

https://www.youtube.com/watch?v=qnL63lEceDEYouTubeYouTubeYouTube

https://voat.co/v/QRV/3679875

Search for "D.U.M.B.S – Special Qperations - UndergroundWar" on Telegram

I have had my ear to the ground during this Ukraine conflict.

Bakhmut and Soledar, as well as Paraskovievka have a vast network of salt mines beneath them. the networks are so extensive, that they were tourist attractions in Victorian (Tsarist) times. Occasionally one hears that certain groups had hidden military hardware and weapons in the tunnels, and then radio-silence. Sometimes there is a news item of Wagners going in and 'clearing the tunnels' but it comes in tiny mentions.

Ugledar, or Vugledar, depending on which language one speaks, is sitting on a coal mine, bordering onto a saltmine further North.

Previously there were similar stories about the Azovstal factory in Mariupol.

One does wonder why all the major battles are fought over tunneled places.

This website has some far out stuff and there is a section on D.U.M.B.s. theorionlines.

https://www.genedecode.org/programs/cabal_and_bloodlines Gene Decode does a lot of interviews on Deep Underground Military Bases. He has a channel on Rumble. https://www.genedecode.org/

Pictures at https://revolutionoftruth.com/dumbs-child-trafficking/ Special Forces Clearing DUMBs Boring Machine Draining the Swamp MAGLEV Trains 17,000 mph Denver Caged Children Rescued DIA DUMB Tunnel Entrance