In order to combat better-supplied American and South Vietnamese forces during the Vietnam War, Communist guerrilla troops known as Viet Cong (VC) dug tens of thousands of miles of tunnels, including an extensive network running underneath the Cu Chi district northwest of Saigon. Soldiers used these underground routes to house troops, transport communications and supplies, lay booby traps and mount surprise attacks, after which they could disappear underground to safety. To combat these guerrilla tactics, U.S. and South Vietnamese forces trained soldiers known as “tunnel rats” to navigate the tunnels in order to detect booby traps and enemy troop presence. Now part of a Vietnam War memorial park in Ho Chi Minh City (formerly Saigon), the Cu Chi tunnels have become a popular tourist attraction. (History.com)