Thanks for A2A, Joshua Alexander.

I had to do a serious research due to my limited knowledge about this topic. The more I read, the more I am amazed by what the previous Vietnamese generations did during the war. In this answer, I would only analyze the Cu Chi tunnel system - the most famous tunnels in Vietnam War.

So if you think of Cu Chi tunnel as comfortable rooms where you can do whatever you want underground, you are wrong. We didn’t have that kind of luxury. Remember you are fighting Americans, the most powerful army in the world, with all kinds of aircrafts and technologies actively searching for you day and night. Imagine if you got caught, not only you are dead but also a part of the tunnel is found, which might cause greater dangers to other people in the whole tunnel system. To put it bluntly, you have to make use of the terrain, work super hard, be extremely resilient, and outsmart your enemy to survive. So what is the tunnel system like and how did they build it?

(No, that’s not how Cu Chi tunnel’s like. This is like a 5-star hotel already.)

(That’s how an entrance looks like.)

The Cu Chi tunnel system was originally built in late 1948 or early 1949 during Indochina War with the aim of maintaining communications between villages and evading French’s sweeps in the area. The Cu Chi tunnels gained its strategically important role in the wars because of its location which is only 40–50 km from the center of Saigon, the most important city in South Vietnam. In the beginning, almost every family had their own secret bunker to hide the Viet Minh soldiers. However, once a separate bunker is detected by the enemy, the soldier could be easily captured or killed as he had only one way out. Hence, there came the need of connecting these bunkers together so that the Vietnamese soldiers can have more chances to escape.

During the Vietnam War, the system was continuously expanded and upgraded to a 250 km system with three levels of tunnels: 6m, 8m, and 10m deep underground. During the day, the peasants farmed on the field but when the sun sets, they started excavating tunnels. In the beginning, for security reason, each village dug their own separate tunnels and nobody knew where the tunnels of other village were. Some of the tunnels are even under the river. The design of the tunnels is zigzag, which served as the link between villages and made it hard for the enemy to explore/travel inside the system. Dust and soil was spread out carefully by being dumped in bomb craters or the river.

Thanks to the tunnels, the Viet Cong guerrillas would ambush American soldiers in the jungles and then disappear without a trace. However, these secret tunnels were not only fortifications for them, but were also the center of their community life. Hidden beneath the destroyed villages, inside these tunnels were underground kitchens and Hoang Cam stoves, water wells, rest areas (dorms), HQ centers, medical areas/hospitals, food and weapon supply storage, schools, and public spaces where couples could get married. In some parts of the network, there were even theaters where performers entertained people with songs, dance and traditional stories. The most vital part for any tunnels, the ventilation holes, were hidden at the foot of trees, thick grass or termite nests. In order to keep the tunnels from collapsing, the VCs used woods from the jungles and iron stolen from the U.S. Army bases to reinforce the tunnels.

Prior to the involvement of US troops, the South Vietnamese troops were proven to be ‘useless’ in the Cu Chi area by numerous VietCong’s victories. There was a time the VCs boldly held a victory parade right in the middle of Cu Chi town. This is one of the reasons Westmoreland needed US troops in Vietnam. However, in the beginning, little did the Americans know that they built their base right on top of an existing network. It took them couples of months to understand why many of their ammunitions, weapons, and food got stolen and they often got shot at night.

But that’s not the best part about the Cu Chi tunnel system. If so, the Americans would have found and destroyed it quickly. Once figuring out there were troops right under their nose, the American and Australian troops, of course, tried a wide range of methods to destroy the whole system and the enemies. The most popular ones include using poisonous chemical gases, “tunnel rats”, and German shepherd dogs.

First, aside with Napalms dropped on the ground, poisonous chemical gases were also pumped into the tunnels. However, the VCs used trap doors with water locks and air locks to stay safe and sound in the tunnels.

Second, the “tunnel rats” were the US soldiers sent down into the tunnels. Of course, they were well-trained and brave, some were Specorces, which VietCong fighters wanted to avoid the most. In order to do so, the tunnel was designed small enough for only Vietnamese people, but very narrow for Americans. The entrance holes in the ground are barely wide enough for Americans, following by a lot of U turns that lead to the surface, then twist again before heading off horizontally further. The tunnels were also very dark and there was no room to return around and retreat. Besides, booby traps were used as many as possible, not only inside the tunnels but also on the ground. Not to mention the intense heat with the lack of air underground (especially when the tunnels are too narrow for their movements) made their job more difficult. In the end, they suffered quite high casualty rates.

Third, the German dogs were undoubtedly dangerous because they could detect both humans and the tunnels. However, the VCs made use of stolen American soap so that the shepherd dogs would identify them as friendly. Captured, stolen US uniforms, belongings were put out to confuse the dogs as well. Most importantly, the dogs were only able to spot humans, not booby traps. Leaving the scent in booby traps could disable the dogs easily so that their handlers would be too horrified to continue the search and stop using the dogs.

Below are some boobytraps used in Cu Chi area, which caused tremendous casualties to the US troops.

Given all that information, you might have had an idea how difficult the life was in the tunnels. More precisely, it was extremely difficult. Air, food and water were scarce and the tunnels were infested with ants, poisonous centipedes, scorpions, spiders and vermin. Like Americans, Viet Cong guerrilla fighters were not super humans so they were scared of these poisonous insects and bugs, too. They were also under the constant threats of American both underground and on the ground. Along with continuous “search and destroy” operations, random artillery was fired into the area at night, and US pilots were told to drop all the unused bombs and napalms there before returning to base. According to some statistics, half a million tons of bombs were dropped on Cu Chi area during the war. I heard that there were times the guerrillas had to stay in the tunnels for weeks or months, laying down on the floor just to get enough air to breath, and many even lost their consciousness or suffocated. Worse, many had to eat stale rice, grass or drink urine to survive. Though only 6,000 out of 16,000 Viet Cong guerrillas serving in Cu Chi survived the war, they didn’t surrender and fought bravely until the end of the war. After 1975, the villages of Cu Chi have been presented with numerous honorific awards by the Vietnamese government, and many of them have been declared ‘heroic villages’.

In early 1970s, the US used B52s to carpet bomb the whole area and destroyed most of the tunnel system. But that was the time when the US troops were about to withdraw from Vietnam, it’s safe to say the tunnels had served their purpose. Today, a part of the existing tunnels was preserved and enlarged for the tourism purpose.

I also found a great documentary with more details and visualization about the Cu Chi tunnels. It was produced by Mickey Grant and the interviewees are former Viet Cong fighters, so its quality and accuracy can be trusted.

I hope this long post has answered your question.

Image sources: various from the Internet.