The Tunnels of Cu Chi (32 page)

Another advanced device in the war against the tunnels was ADSID—the Air Delivered Seismic Intruder Device. The Communists called it the Tropical Tree. This was a small device, three feet high and six inches broad, which looked like a bomb. When it was dropped from a plane, it buried itself in the ground and threw up a four-foot antenna, which was camouflaged like jungle foliage. It transmitted seismic intelligence—footsteps, truck rumblings, and so on—and had a battery life of about one month. The coded transmissions were relayed through aircraft in the area to the signals reception base. The local commander, on hearing activity in a suspected area, could then order an artillery or air strike against the Communists who had strayed noisily above ground.

Captain Linh said that both the Tropical Tree and an earlier American personnel detector
had
caused problems for the men in the tunnels. “The Americans took advantage of our weaknesses,” he said. “We often gathered together outside the tunnels to sing; it was a characteristic of the revolutionaries. We did not have salary, only high spirit. We thought we would be safe at night, but it was not so.” At first the Americans used a primitive Mark One version, claimed Captain Linh, a boxlike radio transmitter dropped by red-and-blue parachutes. But it had a short battery life and was easy to spot and destroy. The Tropical Tree, however, because of its camouflage and long battery life, was a problem at first. On one rice-collecting mission, one of Linh's tunnel squad, having picked up the rice, loitered by a surface well to wash and lark around a bit in the fresh air. Within minutes, he came under heavy and accurate artillery fire. The man escaped, but the area was devastated. “I couldn't understand why this had happened, and I left the place alone for several weeks after that,” said Linh.

The next time the Viet Cong returned to that spot, Linh personally led his men, creeping slowly through the jungle, quietly clearing new paths. The mission was a success. It was dusk, and once again it seemed perfectly safe to relax for an hour or so. The men boiled some water for tea and talked loudly and laughed while they drank. Almost at once an American spotter plane flew over, followed within minutes by an airborne attack. Linh and his squad raced to the nearest tunnel entrance; he was last in, and within moments, a bomb fell on the tunnel above his head. Linh lost consciousness, but because the bomb failed to penetrate, he didn't lose his life. When he was revived, he took his men through the tunnels to a spot about one kilometer away and they watched the air strike continue for several hours. When it was over, they went back to inspect the damage and found their first cluster of Tropical Trees—old ones uprooted in the bombing strike, and mostly inactivated because of it.

Linh took the devices back to Phu My Hung, where they were examined to see how they worked. The Viet Cong figured out that if the Tropical Trees could be quietly approached, and the four or five sprouting aerials tied together, this rendered the transmitter ineffective. This is how they were eventually neutralized—but only if the bombwatchers, who also looked out for unexploded ordnance, were sufficiently adroit to spot where the ADSIDs fell.

The Americans did play one tactical trump card in this game. As described by tunnel rat Lieutenant David Sullivan: “It worked rather well. We pretended to ‘lose' PRC-25 radios from helicopters as they were taking off with a dust-off or whatever. VC trail-watchers inevitably saw them falling out. In fact, these radios were deliberately thrown out and they had specially bugged antennae. Often the VC would take the bugged radios back to the tunnels—and this led to several tunnel discoveries.”

But this was the height of technical sophistication compared with some of the other gadgets—gizmos, as the Americans called them—that came from the crowded test-benches of laboratories in the United States. Project Bedbug, sponsored by the Limited War Laboratory (LWL) in Maryland, only just failed to take off. This involved the tactical use of bedbugs (or “man-seeking anthropods,” as LWL called them) to warn infantrymen of approaching VC guerrillas. William Beecher of
the
New York Times
, who visited LWL, described the project as follows: Since bedbugs “let out a yowl of excitement when they sense the presence of food, specifically including human flesh, the lab created a bedbug carrier fitted with a sound amplification device.… when a bug-bearing patrol approached an enemy ambush, the members of the patrol would be forewarned by the happy cries of the animals upon sensing a meal up ahead.” The project collapsed when it was discovered that the bedbugs couldn't control their excitement. They became so deliriously happy at just being carried about by GIs that they were too busy “swooning with delight” to warn their patrons of any approaching Communist ambush.

By 1967, there were so many tunnels in the Cu Chi area that any thorough search was bound to find new entrances. Lieutenant Jerry Sinn, who led one of the two tunnel rat squads in the Big Red One, calculated that during Cedar Falls the Rome plows were turning up thirty to forty tunnel entrances in every square kilometer of the Iron Triangle. But this did not deter the scientists back in Maryland from inventing something brand-new. They now came up with the Tunnel Cache Detector—the Portable Differential Magnetometer. During tests in the United States it seemed to work well. It was rather like a metal detector and allegedly spotted the soil characteristics where tunnels had been dug. The sensor then gave an aural warning to the operator. Theoretically, all you had to do was carry the pack around, set it up, stick the sensor in the ground, wait for a reading—and bingo. At least that was the intention.

However, the more the tunnel rats began to understand tunnels, the less they needed equipment. Nevertheless, the Portable Differential Magnetometer (PDM) arrived in Cu Chi for evaluation, weighing 106 pounds. It was brought by NETT, the LWL New Equipment Training Team, comprising two NCOs from Maryland and one representative of the manufacturer. Training started in the old VC tunnels on Cu Chi base, and all the trainees said they understood how to work the PDM after twelve hours. Next came operational training. Some problems developed. The weight and physical awkwardness of the PDM made it extremely difficult to get through undergrowth. One operator who tried carrying it in typically heavy vegetation became exhausted and collapsed and was left behind by his own men.

In other combat tests the unfortunate operators found it took so long for the cumbersome machine to be set up, tested, and then operated that the supporting infantry squad, whose job it was to provide security for the operator, simply moved on and out, reluctant as ever to loiter in VC-controlled territory. Once left alone without infantry cover, the operator tended to end the test prematurely and run after his bodyguards. As testing continued, the use of PDM decreased so dramatically that it averaged less than one operation per unit per week. In the whole of its evaluation trials, the PDM discovered precisely one tunnel—just ten feet long, and abandoned. And it found this only because the unit had originally spotted, with their eyes, a nearby important tunnel complex. It was during this operation that the operator encountered many false signals on his PDM from fragments of metal embedded in the soil (as they were all over Vietnam). The many canals, ditches, and rice paddy fields in the Cu Chi area further restricted the size and shape of any attempted search pattern. Five of the eight PDMs broke down, and the crucial sensor heads and rotating joints “continually became loose.” Unit commanders and tunnel rats pointed out that most tunnels really
were
spotted by the old-fashioned eyeball, and the LWL team went home. The PDM never was deployed in Vietnam.

Undeterred, the scientists back home next came up with the Seismic Tunnel Detector (ACB-35/68M)(U). Here was yet another method, this time using the sonar principle of sound detection, to do electronically what the tunnel rats were doing with their eyes and by touch. The STD made even the PDM look like a diviner's twig. It needed two operators to cart it around, one to handle the probes that were stuck into the ground, and one to act as mulepack for the heavy electronic gear that went with it. Once again, initial training began at the 25th's Tunnels, Mines, and Booby-Traps School, inside Cu Chi base. Once again, it was taken out on a combat evaluation.

The evaluators, flown in from Washington, discovered that in dry soil it took one full hour to poke around just one fifty-square-foot area. Time and again the coupling between the gizmo and the ground either failed or took so long that the infantry unit accompanying the evaluators prudently took to their heels, rather than wait as sitting targets for tunnel snipers. Left on their own, the two-man crew had “difficulty” in rushing
through the jungle, given the size and weight of the—so far—useless machine. The Seismic Tunnel Detector never made it back to Vietnam either.

Tunnel detection was to remain an overrated problem while tunnel destruction was consistently underrated. It is ironical that in the one field in which high technology really
was
needed in order to secure tactical military victory in the tunnels war, it was never seriously applied. There was little scientific investigation of the tunnels structure or comprehension of the vital role of the interlevel trapdoors, which acted as efficiently as the conning-tower hatch does for the submarine. There was a reluctance by units to stay on the ground overnight, unless on a major military operation like Cedar Falls, in order to complete the work of tunnel destruction. And ultimately there was a misunderstanding at the highest command level of the true role of the tunnels of Cu Chi. All these factors allowed the Communists to keep their tunnels virtually intact for a long time.

Lieutenant Colonel James Bushong, the chemical officer with the 25th Infantry, admitted that they never discovered any effective way to destroy the tunnels or even deny their use to the Communists. He still does not know how he would do the job today, without using banned chemical weapons. Brigadier General Richard Knowles, who agreed that the importance of the Cu Chi tunnels system was underestimated, felt the nature of the war itself precluded the kind of planning that would have led to their destruction. He was the commander of the 196th Light Infantry Brigade, and he directed and fought a highly mobile war, much of it using the ubiquitous helicopter. “One of the problems with exploring these tunnels was the fact that you'd park all this sophisticated equipment and leave a highly exciting and fast-moving situation to go into a dark tunnel that was quiet and took a lot of time and patience and was very tedious, and that's just not in the American character, to take to that sort of thing naturally. Commanders like to do the things that are exciting, that the men like, and you get recognition. You couldn't get reporters to go down those holes, and it just wasn't of interest to most Americans.

“After Cedar Falls I was fully aware of the tunnels, but certainly not of the significance of the VC and NVA effort in South Vietnam.… I'd like to feel that we appreciated the full
significance of the tunnels at that time, although the technology wasn't available [for their destruction] … I'm convinced we could have done it. But we didn't have the patience, the time; there appeared to be too many other things that were lucrative that drew us away.”

Because of the relative proximity of the Saigon River to many of the tunnel systems, flooding was thought to be an easy and convenient method of tunnel destruction. In fact, it turned out to be an expensive, time-consuming failure. On 3 December 1967, the 1st Battalion, 277th Infantry, was assigned the task of destroying a large complex that had been uncovered eleven days earlier. Flooding was the chosen method, after water sources relatively close to the main tunnel entrance had been located. Firstly, a bulldozer had to clear a path through the jungle from the tunnel mouth to the water source. Because of a shortage of flexible hose it became necessary to lengthen the canal, which was being used as the water source. That being done, a ditch had to be constructed, again with the bulldozer, from the tunnel entrance to the end of the (still short) flexible hose At the water source end, because of the sheer toughness of the soil, the extra ditch had to be blown out, using bangalore torpedoes (five-foot-long torpedo-shaped explosive charges). Two hundred and fifty meters of ditch had to be blown out this way.

Next, two large water pumps and a further 3,000 feet of flexible hose had to be flown in by helicopter from the Cu Chi base. As the installation of the pumps and the hose presented further problems, the 1st Platoon, A Company, of the 65th Engineering Battalion had to be flown in to help. By now, the area was beginning to resemble the foundation-laying of the Boulder Dam. The bulldozer was used to lay the extra hose along the ditch it had previously cut. Finally, the pumping began, and for the next thirty-eight hours, 800 gallons of water were pumped every minute into this one single tunnel complex. However, the pumping completely failed to destroy it—the water just drained away. So in desperation, cratering charges were floated in and detonated. Even then, only some of the tunnel was destroyed.

Captain Herbert Thornton found flooding tunnels a complete waste of time. “Even if we'd diverted the entire Saigon River or a complete rice paddy into the tunnels, it wouldn't have
worked,” he explained. “The problem was the clay laterite absorbed the water in the dry season. In the wet season it didn't matter, because those trapdoors held fast anyway.” Colonel Thomas Ware, a former battalion commander in the 25th, also found flooding was useless. “It maybe inconvenienced them for a while, but the tight locks worked. Charlie always came back.”

The militarily romantic but technically inept gesture of hurling grenades into tunnels in the belief that they somehow made them collapse was also primarily an exercise in naïveté. But the system was frequently used by non-tunnel rats who had no wish to be involved in tunnel exploration, or by soldiers in a hurry to move on and out. Grenades made a lot of noise and dust and were effective killing instruments at close quarters, but trying to destroy tunnels with them was like scooping out a river with a teaspoon. Cratering charges, C4 charges, and Shape explosive charges were also commonly used. Tunnel “denial” was often assumed after these explosions, but there was insufficient postexplosive investigation to establish the true extent of the damage.