The Cu Chi Tunnels are seventy five miles of interconnecting tunnels which were employed by the Vietnamese army during the Vietnamese War (or American War because they contact it). They served as hiding places and as routes for communication and materials. Quite a few troopers even lived in these tunnels https://saigon96037.blogminds.com/the-fact-about-sai-gon-list-that-no-one-is-suggesting-28010264