The Củ Chi Tunnels (Vietnamese: Địa đạo Củ Chi) are a vast network of interconnected tunnels in the Củ Chi District of Ho Chi Minh City (Saigon), Vietnam, and are part of a much larger tunnel network that crisscrosses much of the country. The Củ Chi Tunnels were the site of several military campaigns during the Vietnam War and formed the Viet Cong's base of operations for the Tết Offensive in 1968.
The tunnels were used by Viet Cong soldiers as hiding places during combat and served as communication and supply routes, hospitals, food and weapons depots, and living quarters for numerous North Vietnamese fighters.
Source: Wikipedia