Capitol Siege Shines Light On Vast Underground Tunnel Network
https://www.thedrive.com/the-war-zone/38593/evacuations-during-capitol-siege-shine-light-on-vast-underground-tunnel-network
Listen To What The Capitol's 'Giant Voice' Alert System Sounds Like When It Goes Into Lockdown
https://www.thedrive.com/the-war-zone/38780/what-the-capitols-giant-voice-alert-sounds-like-when-it-goes-into-emergency-lockdown
2017
"Late last year, the Army launched an accelerated effort that funnels some $572 million into training and equipping 26 of its 31 active combat brigades to fight in large-scale subterranean facilities that exist beneath dense urban areas around the world."
https://www.military.com/daily-news/2018/06/24/army-spending-half-billion-train-troops-fight-underground.html