Tunnels Of Củ Chi, Ho Chi Minh City, Saigon, Vietnam

From its mistakes, and the Australians' discoveries, U.S. command realized that they needed a new way to approach the dilemma of the tunnels. A general order was issued by General Williamson, the Allied Forces Commander in South Vietnam, to all Allied forces that tunnels had to be properly searched whenever they were discovered. They began training an elite group of volunteers in the art of tunnel warfare, armed only with a gun, a knife, a flashlight and a piece of string. These specialists, commonly known as “tunnel rats”, would enter a tunnel by themselves and travel inch-by-inch cautiously looking ahead for booby traps or cornered PLAF. There was no real doctrine for this approach and despite some very hard work in some sectors of the Army and MACV (Military Assistance Command, Vietnam) to provide some sort of training and resources, this was primarily a new approach that the units trained, equipped and planned for themselves.
Despite this revamped effort at fighting the enemy on its own terms, U.S. operations remained insufficient at eliminating the tunnels completely. In 1967, General William Westmoreland tried launching a larger assault on Củ Chi and the Iron Triangle. Called Operation Cedar Falls, it was similar to the previous Operation Crimp, however on a larger scale with 30,000 troops instead of the 8,000.
On January 18, tunnel rats from the 1st BN 5th Infantry Regiment, of the 25th Infantry Division, accomplished the uncovering of the Viet Cong district headquarters of Củ Chi containing half a million documents concerning all types of military strategy. Among the documents were maps of U.S. bases, detailed accounts of PLAF movement from Cambodia into Vietnam, lists of political sympathizers, and even plans for a failed assassination attempt on Robert McNamara.
By 1969, B-52s were freed from bombing North Vietnam and started "carpet bombing" Củ Chi and the rest of the Iron Triangle. Ultimately it proved successful. Towards the end of the war, the tunnels were so heavily bombed that some portions actually caved in and other sections were exposed. But by that time, they had succeeded in protecting the local North Vietnamese units in "surviving to fight another day".