A man is lying on his back in total darkness 10 m under Cu Chi on a night in the spring of 1966. And directly above his head, an American generator is running. He can hear it through the clay. A steady diesel hum, a little louder than his own breathing. He has been down here for 4 days. He has not seen the sun or a lamp or his own hands.

The air is warm and wet and tastes used because it has already been through the lungs of the men lying beside him in the same passage. A passage by most accounts about a meter high and less than a meter wide, dug by hand and shored with nothing. Then there is a second sound, and this one he cannot place. Something above him yelps once, high and sharp.

 And the yelping moves away fast across the ground and is gone. Tonight he’s going up. Not out into the jungle, up through a wooden trapdoor inside the wire of the base camp [music] of the United States 25th Infantry Division, into a camp of thousands of American soldiers to do what he has already done more than once and come back down. The men on the roof of his tunnel do not know where he comes from.

By the end of this video, you will know why they never could. Hold three things for the next few minutes. >> [music] >> First, a captured Viet Cong report puts a number on how many men in a tunnel unit were sick at any given time, and the number is not a percentage anyone would have accepted in an American company.

Second, >> [music] >> there is a stove down there invented by a cook 15 years before the Americans arrived. And the reason it never gave anyone away is the same reason the trapdoor inside the base was never found. Third, in January 1967, the Americans launched 30,000 men at these tunnels. And the paper count they came home with is a number with six digits.

And it answers the question of who was besieging whom. This is how Viet Cong fighters ate, slept, and survived for years under an American base. Start with the roof. In January 1966, the 25th Infantry Division arrived at Cu Chi, about 30 km northwest of Saigon. And in February, [music] engineers began building one of the largest American bases in Vietnam on top of it.

Flat ground, good drainage, room for airstrips and helicopter pads. Nobody surveyed what was under the ground because by most accounts, nobody knew there was anything to survey. The base had been open only a short time [music] when the strange losses began. Helicopter motors damaged overnight. Food gone from stores.

Soldiers found killed in their cots inside a guarded camp. Chinook helicopters wrecked by saboteurs who had never breached [music] the perimeter because there was no record of anyone crossing the perimeter at all. For several weeks, the division’s rear areas, the safest ground on the map, were attacked by Viet Cong who came up out of the earth.

And the men who wrote the reports afterward >> [music] >> reached for a phrase the army did not have a doctrine for. An envelopment from below. The truth, when it came, was worse than the phrase. The whole camp was undermined. Passages and trapdoors ran under the airstrip, under the tents, under the motor pool.

 [music] There was a tunnel with a trapdoor inside the American base itself at Dong Du. The man in the cold open did not have to sneak past the wire. He was already inside it. And the base stayed. The 25th division kept its headquarters at Chu Chi from January 1966 >> [music] >> until February 1970. On the 3rd of January 1969, a Viet Cong bomb went off in a mess hall on the base and killed 17 men.

On the 26th February 1969, sappers got onto the base and destroyed nine CH-47 Chinook helicopters. The Americans left in 1972 >> [music] >> and the camp passed to the South Vietnamese 25th division. For roughly 5 years then, there was an American city on the roof and there were people living in the basement.

 [music] Now, the basement. How did they eat, sleep, and survive down there? The rhythm of the whole life was day below [music] and night above. Soldiers spent the daylight hours in the tunnels working or resting and came out only at night to scavenge supplies, tend crops, or fight. That was the good version.

 During heavy bombing or when American troops were moving through the area, they stayed under for many days at a time. When Thi Nia, who joined the revolution at 13, told Agence France-Presse how her village went underground and that she once spent 5 days in a hot, claustrophobic tunnel. 5 days is one woman’s memory of one stretch.

Duong Thanh Phong, a photographer from Saigon, went down with his cameras and recorded the war from inside the tunnels for 5 years, documenting the civilians and the [music] soldiers of Cu Chi. That is where the number in the title comes from. Not a unit average. One named man, five years in the dark. Eating meant fire, and fire meant smoke, and smoke above a tunnel was a [music] death sentence for everyone under it.

The answer was a stove. It is named for the man who invented it, Hoang Cam, a Viet Minh cook during the Hoa Binh campaign of 1951 and 1952, a full 14 years before the 25th Division landed. The cooking fire sat in a deep covered hole. From it ran long bamboo vents underground, and the smoke was [music] pushed through those channels and filtered by the earth around them.

 So that when it finally reached the surface, it rose in thin, low wisps, spread out over a wide area, and looked like nothing more than morning mist over the fields. No column. Nothing for a spotter plane to fix on. The Hoang Cam stove was used all through the Cu Chi tunnels and along the Ho Chi Minh Trail. And its inventor lived on quietly, a captain when he left the army in 1958, and died in 1996.

Hold that stove in your mind, because its logic is the logic of the whole system. Nothing that leaves the ground is allowed to look like a man made it. Sleep was a body on packed clay in [music] a passage where grown man could not stand. Communication tunnels, by most accounts, were dug at roughly 0.8 to 1.2 m high.

The sections tourists crawl through today, about a meter high and less than a meter wide, have been enlarged to make them accessible, which means the originals were smaller than anything a visitor is now allowed to experience. Men slept where they stopped. In the bombed areas, people spent much of their lives underground, and Cu Chi grew into entire villages under the earth with living quarters, kitchens, [music] workshops that made weapons, hospitals, and bomb shelters.

 A network that by most accounts linked bases across some 250 km from the outskirts of Saigon to the Cambodian border. Now, the first thing I asked you to hold, sickness was everywhere, and malaria was the second largest cause of death in the tunnels after battle wounds. A captured Viet Cong report stated that at any given time, half of a unit had malaria, and that 100% had intestinal parasites of significance.

Read that again. The document was not written by an enemy trying to make them look weak. [music] It was their own report on their own men. Half the unit shaking [music] with fever, every single man carrying worms. Those are the soldiers who came up through the trap door at night, killed men in their cots, and went back down to lie in the dark until the fever passed.

And the dark was total. There’s no other word for it. A lamp costs oil and oxygen, and both were rationed by the earth. Men lived by touch, moving along walls they knew, [music] past turns they counted. Dương Văn Phong, after 5 years down there, >> [music] >> surfaced at least once and could not find his way back to the entrance he had just climbed out of.

That is not a man who was lost. That is a man who knew the tunnels better than [music] almost anyone alive. Standing in daylight, unable to find a hole he had used for years. Remember that, because it is the answer to the question I opened with. And the Americans were looking for that hole with 30,000 men. So, the second question.

Why could they live down there at all? Because the tunnels were not one tunnel. By most accounts, the Cu Chi [music] tunnels ran on three levels. The first about 3 m down, the second at 5 or 6 [music] m, the deepest at 8 to 12 m, 10 m down, more than 30 ft. And that lowest level held the hospitals, the weapon stores, and the command [music] posts.

One source puts the deepest level at 36 ft. Nobody surveyed it with instruments during the war, so the honest range is 8 to 12. Depth alone does not stop gas or water, and the Americans tried both. What stopped them was plumbing. Passages between levels were built with U-bends, the same trick as the pipe under a sink, so that a section could be flooded with water, and the water would sit in the bend and go no further.

And gas pumped in at an entrance would meet a sealed trapdoor and a water trap >> [music] >> before it reached anyone. Trapdoors were cut to fit flush and covered with earth and leaves, and a man on the surface could stand on one and not know it. The ventilation was hidden the same way the stove smoke was, through small holes disguised in termite mounds and undergrowth.

 [music] Then there were the dogs. The Americans brought tracker dogs to find the vents [music] and the entrances. And this is the second sound in the cold open, the yelp that moved away across the ground. Colonel Chow Lam, a Viet Cong tunnel veteran, known by the code name Uncle Number Seven, told NBC News in 2003 exactly what they did.

We put hot chili peppers and garlic and all kinds of spices in those areas. So, once the dog smelled it, it would destroy their nose and they’d go yelping away. A dog with chili in its nose is a dog [music] that finds nothing, and its handler walks past the trapdoor with a man lying under it. You have probably heard about the traps, and some of it is true.

Pits with spikes, springboards, grenades on tripwires. What you may also have heard is a legend about a snake so venomous that a man bitten by it dies within three steps. That kill speed story is false. Traps were real. The three-step snake was not. And any account that leans on it should make you check everything else it says.

And the thing that matters most is that none of this was built for the Americans. The tunnels predate them by about 20 years. They were begun in the war against the French, dug and extended village by village, and the men who eventually taught Americans what they were up against had been living with them for a generation.

Nam Toan, Communist Party Secretary of Phu My Hung Village, Six Hamlets, and political commissar of its defense force, is described by Tom Mangold and John Pennecate as leading a village platoon that had not spent years inside the tunnels like the tunnel soldiers, but had fought many battles in their dank blackness.

Even the part-timers had a tunnel war behind them. Now the flip and the third number. Operation Crimp ran from the 8th to the 14th of January 1966 >> [music] >> in the Ho Bo Woods, about 20 km north of Cu Chi. American and Australian troops went in expecting a headquarters and found a network that by the operation’s own count covered more than 200 km.

And they came out with roughly 100,000 pages of documents. That was the first time anyone on the American side understood the scale. The first American tunnel team was set up that year by Captain Herbert Thornton, >> [clears throat] >> a chemical core officer of the First Infantry Division, whose job was contaminating tunnels with chemical agents, and who, according to the National Museum of the United States Army, picked his men for even temperament, >> [music] >> an inquisitive mind, and exceptional bravery.

A year later, the Americans came back with everything they had. Operation Cedar Falls, the 8th to the 26th of January, 1967, sent around 30,000 troops, two army divisions, an infantry brigade, and an armored cavalry regiment into the Iron Triangle, 20 km north of Saigon. On the first morning, the First Battalion, 26th Infantry, came into the village of Ben Suc by helicopter assault.

About 6,000 villagers were forcibly relocated, and the village was destroyed. Over 19 days, the Americans destroyed 1,100 bunkers, >> [music] >> 400 tunnels, and 500 other structures, and captured 3,700 tons of rice and over 600 weapons. Communist losses were about 700 [music] dead and 700 captured. And the paper count was 491,553 pages of documents.

Only after Cedar Falls did tunnel rat duty in the First Infantry Division pass to the First Engineer Battalion, >> [music] >> where men began to specialize in it. Read those numbers the way the men who wrote them read them. Nearly half a million pages, 30,000 troops, a village erased, and under it all, the same tunnels, [music] the same trap doors, and the same base camp at Cu Chi, still being attacked from below 2 years later, still losing 17 men in a mess hall in 1969.

Still losing nine Chinooks on a single [music] night. The Americans believed they were laying siege to the tunnels. The tunnels had been laying siege to the base since the day the engineers broke ground. Chau Lam put the tactic in one breath. >> [music] >> From the tunnels we were able to fight Americans from every angle.

They didn’t know where we were coming from. That way we could do more damage and limit casualties with fewer forces. And on what it cost, he did not smooth it. We didn’t want to kill American soldiers, but we had to just to make them leave the country. So, you have the three things. Half a unit with malaria and all of it with parasites, and they fought anyway.

A cook stove from 1951, whose whole design was that nothing leaving the earth should look human. And the trapdoor inside the base obeyed the same rule. And 30,000 men, nearly half a million pages, and a base that was still being hit from underneath 2 years after the biggest operation of the war so far. The Americans thought they were besieging the tunnels.

 It was the other way around. The man from the cold open is one of the ones the records never name. Dương Văn Phong is one they do. He went down in the dark with a camera, stayed 5 years, and photographed people eating by stove light and sleeping on clay >> [music] >> while a division ran its generators overhead. Then one day he climbed out into sunlight, turned around, and could not find the way back in.

If he could not find it, nobody on the roof ever had a chance.