The Tunnels of Cu Chi by Tom Mangold Page A

pumping down a large quantity of water. But this was not so. It was important that the enemy never understood this.”
    In reality, this claim is a mixture of boast and fact. In the early days of tunnel warfare the Americans were certainly not aware of the secret exits. But it soon became obvious that trapped VC were not always being caught at tunnel entrances that were sealed by the attacking GIs. The only possible explanation was clearly the existence of hidden escape routes, which the Americans found only with considerable luck or ingenuity.
    Although the tunnels were natural shelters against the U.S. bombing attacks, further special protection became necessary when the bombing increased in ferocity. So the tunnelers dugconical A-shaped shelters that were geometrically designed to resist both artillery shells and bomb blast. More important, their conical shape acted as an amplifier and magnified the distant sound of approaching B-52 strikes. This was the only real warning tunnel dwellers might get of an imminent attack.
    Ultimately, the real security for the tunnel system depended on the precise and cunning use of camouflage. An undated captured VC document advised Cu Chi cadres as follows on the subject:
    If the duration of use [of the tunnel] is long, we should grow viable plants. Change dried leaves before [they get] dark, and blot out all suspicious traces before daybreak. Give a contrast to the camouflage by using high and low plants. Do not show a dull and prominent heap of earth. Plants and branches must be picked far from the fortifications and the troop’s locations. When an emergency repair is necessary never pick up branches and leaves between the enemy and us, and especially do not gather a great number in one place.
    An informant, code-named TU 10, was placed by American military intelligence somewhere in the Tay Ninh area from January to July 1967. On 28 July 1967 he spoke to his controllers at some length about the continuing success of VC tunnels camouflage, revealing some significant things about U.S. infantry technique when it came to tunnel warfare:
    Experience had shown the VC cadre that when friendly forces discovered any sort of hidden place they were inclined to destroy it and move on without further intensive search … tunnel and cache entrances located in residential areas were often placed under a cooking place, and if feasible, in a pig pen, the latter being more desirable because Americans hesitated to look in such places. A large corner post of a building or roofed animal shelter would sometimes be used to conceal the entrance to a tunnel or cache site.
    Source TU 10 reported that it was a general rule (although not always adhered to) that foliage used in camouflage must bechanged every three days. Often efforts were made (he did not say with what degree of success) to use “small bushes with many leaves” as camouflage, and where possible, living plant shoots, which would grow and remain green.
    But Ngo Van Giang, the VC prisoner who had given U.S. intelligence useful information about tunnel construction, suggested there
were
ways for the Americans to discover tunnels, despite the excellent camouflage. Properly trained eyes could see evidence of trails, tree branches might be broken in the area, grass on both sides of the trails might be crushed, there might be several unexplained high knolls in the area, and in the tunnels area the dirt would be slightly more spongy.
    The highly trained American Special Forces soldiers, the Green Berets, were specifically trained to break through even the most adroit camouflage systems. They lived rough, often in VC-dominated areas, and became renowned for their field and combat skills. Several were attached to infantry units that went out on search-and-destroy operations, and the Green Berets’ ability to spot tunnel entrances was exceptional. But there were never enough Special Forces liaison NCOs to go round, and, if given the