The Cu Chi Tunnels are 75 miles of interconnecting tunnels which were used by the Vietnamese armed forces through the Vietnamese War (or American War as they call it). They served as hiding places and as routes for conversation and materials. Many troopers even lived in these tunnels for durations https://saigonlist12581.tribunablog.com/not-known-factual-statements-about-sai-gon-list-42382106