The Cu Chi Tunnels are the most popular day or half day excursion from Ho Chi Minh City. Full day tours begin in the early morning and head first of all to the Cao Dai Temple at Tay Ninh on the Cambodian Border. After lunch the tour proceeds to the tunnels returning to Ho Chi Minh City by late afternoon or early evening depending on the traffic entering the city.
The Cu Chi Tunnels are a network of around 250km tunnels built by the Viet Cong over a twenty year period which allowed them to move unchallenged underneath enemy held territory including the US military base at Dong Du. At one time they allowed access to the outskirts of Saigon from as far away as the Cambodian border.
Originally the tunnels had been built in the 1940s in the fight against the French as they allowed underground communication between villages. In the 1960s the VC repaired and expanded them in their fight against the Americans and the South Vietnamese using them not only for communication but also for surprise attacks on the enemy. US ground operations in search of the tunnels lead to the loss of many American lives so a huge bombing campaign against Cu Chi including chemical destruction of the vegetation took place but the VC weren't found to this day crop yields are poor as chemicals remain in the soil and water).
American "tunnel rats" were the next solution the Americans came up with as they sent men into the tunnels to seek out the enemy causing terrible casualties. Eventually Cu Chi became a 'free strike zone' meaning that little (if any) permission was required to carry out any form of attack on the area. Eventually in the late 1960s American B-52s destroyed most of the tunnels and the surrounding area but the tunnels had already served their purpose.
Recommended Reading: The Tunnels of Cu Chi (Tom Mangold & John Penycate)