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

>FORT BRAGG, N.C. — It is darkness like you’ve never seen. The air you breathe could kill you in moments. All of your fire support — air, armor, artillery — is useless. The walls and ceiling could collapse. Communications will fail. A wrong turn leaves you utterly alone.

>Going underground, in dark, tight spaces, can trigger feelings of helplessness in even experienced troops. It is the most primitive and close combat a fighter may face.