Cu Chi Tunnels - Part 2: Strategic Import and Structure

In this second installment about the tunnels of Cu Chi district in Vietnam, we will continue to quote from the book, The Tunnels of Cu Chi (Mangold/Penycate, 1985). This installment will focus on the strategic significance and the structure of the Cu Chi tunnels, which were highly instrumental in the Vietnamese guerrilla fight against the American military machine (for a diagram of a typical tunnel system, see here ... and here).

"On 28 September 1967, a detachment of the Korean 28th Infantry Regiment of the 9th (S. Korean) Division captured a remarkable document during a sweep north of Saigon ... The document appears to be ... the only tunnels manual ever issued by the Communists. It is a ten-page technical and political booklet, revealing many secret details about the tunnels' structure and strategic purpose ...

" The primary role of the tunnels is stressed and re-stressed. 'They are for the strengthening of combat vitality for our villages. They also provide more safety for our political and armed units and for the masses as well. But their sheltering purpose in only significant when they serve our soldiers in combat activities. As mere shelters, their great advantages are wasted.' ... 'There must be combat posts and equipment inside the underground tunnels for providing continuous support to our troops -- even if the enemy occupies the village.' The document [continues]:

'If the tunnels are dug so as to exploit their effectiveness fully, the villages and hamlets will become extremely strong fortresses. The enemy may be several times superior to us in strength and modern weapons, but he will not chase us from the battle-field, because we will launch surprise attacks from within the underground tunnels. We can see that underground tunnels are very favorable for armed forces as limited as ours, in strength and weaponry.'

"The tunnels would be crucial for launching close-in attacks on the Americans and would also provide opportunities to seize their weapons; they would provide excellent mobility and 'we may attack the enemy right in the center of his formations or keep on fighting from different places.'" (p. 56-57)

"[The tunnel] system was to be simple and effective: 'We must plan for the eventual impossibility of fighting from inside the underground tunnels. A secret passage must then be available from which our troops may escape and fight in the open, or reenter the underground passage if necessary.' The passages of the tunnels were not to be either straight or 'snakelike', but were to zigzag at angles of between 60 and 120 degrees ... Zigzagging ... [made] a straight line of fire inside impossible, and helped deflect explosive blasts.'" (p. 57-58)

"A clever and finely engineered trapdoor system was devised ... to create entrances and exists to secret passages and from one tunnel level to another ... Air, sanitation, water supplies and cooking facilities were sufficient to maintain a primitive but reasonably safe existence. It was crucial to the whole plan that even if the first tunnel level was discovered, the secret trapdoor that led down to the next would remain hidden from the enemy. That meant making trapdoors that were virtually invisible." (p. 58)

"The sides of the trapdoor were usually beveled downward at an angle so that it could take considerable over-pressure. There was no sag. If the trapdoor was inside the tunnel, the VC placed earth on top of it and hid in the earth small finger wires, which allowed a soldier to lift the door. If the trapdoor was outside, then small plants would be encouraged to grow on it, or dead foliage would be ... planted to make it as one with the environment.

"Ventilation holes were simplicity itself. They ran obliquely from the surface to the first level -- obliquely to avoid monsoon rain flooding in. Some always pointed east toward the preferred light of the day. Others, 'must be turned toward the wind.'" (p. 59)

"Entrances to the tunnels were carefully and precisely engineered to cater for various contingencies. The ... manual explained:

'Because the activities of the militia and the guerrillas require appearing and disappearing quickly, the entrances to the underground tunnel must be located like the corner of a triangle, so that each can support the other in combat. Our troops must also be able to escape from the underground tunnel through a secret opening so they may continue to fight.'

"The entrances also had to be able to resist fire, flood, and chemical warfare: 'for this reason, we must locate the entrances to the tunnels in dry, elevated, and well-ventilated areas. Such an entrance will not be blocked by the chemicals that will otherwise kill the occupants. Also rainwater will not stagnate in the entrance so located.'" (p. 59)

"[The] fact remains that the tunnels of Cu Chi were the primary factor in fighting the campaign against the Americans, and if sloppiness or engineering imprecision infected the building of the system, the communists would lose.

"Some first-hand evidence of the stability and efficiency of the Cu Chi tunnel system fell into American hands when a VC guerrilla, Ngo Van Giang, was captured by the S. Vietnamese on 31 January 1968. In a sixteen page debriefing statement, Giang is quoted at length by his interrogators on the subject of the Cu Chi tunnel network. He told his captors where a tunnel became an open bunker, special roofs had been constructed by using 50-cm-thick layer of 'husks'. Then there was a layer of dirt 50 cm thick. On top of the dirt, they had planted flowers or used fallen trees as camouflage. Incredibly, according to Giang, if a 200-kg bomb fell within just ten meters of the tunnel, no damage would result. The husks and leaves used were excellent protection against bomb blast. Bamboo poles were also employed for their resilience. 'In April 1966,' Giang told his captors, 'an airplane dropped a 200-kg bomb at Chua hamlet, and the bomb hit right on this type of tunnel. The dirt and husks caved in, but the cadre [inside] was not wounded.'" (p. 60-61)

"One of the most important secrets of kept from the Americans during the entire war, according to Major Quot, was that the construction of the tunnels was such that each section could be sealed off. 'The Americans thought that our armed forces were confined to one tunnel and that they were able to kill everybody down there by blowing down gas or pumping down a large quantity of water. But this was not so. It was important that the enemy never understood this.'" (p. 64)

"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 dug conical 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 warning tunnel dwellers might get of an imminent attack." (p, 64-65)