The tunnels of Củ Chi are an immense network of connecting underground tunnels located in the Củ Chi District of Ho Chi Minh City (Saigon), Vietnam, and are part of a much larger network of tunnels that underlie much of the country. The Củ Chi tunnels were the location of several military campaigns during the Vietnam War, and were the Viet Cong’s base of operations for the Tết Offensive in 1968.

Source: https://en.wikipedia.org/wiki/C%E1%BB%A7_Chi_tunnels