Cu Chi is the area (district? Village?) where, among other things the Americans had a big army base during the war and the VC built an intricate system of tunnels, 200 km of them. The reason was to hide VC fighters, covert regrouping and surprise attacks.
As the US used B52 bombers with 200-450 kg bombs, carpet bombing large areas at a time, the VC built the tunnels in levels. First level was <=3m deep and would not survive those bombs. Entrances, ventilation shafts. Second and deeper level could go down to 15 meters or more. Cooking would only be done at night so the smoke could be mixed with the nightly smog. The US had enrolled south Koreans and Mexicans to clear tunnels as they are smaller, and dogs sniffed out the entrances. First, detection by dog was hindered by peppers and chili but it would still give away that there was an entrance so later corpses to mask scents or American gaments to make the dogs discard the detection.
Traps were built of course, one more ingenious and lethal than the other. It became a game of cat and mouse, eg spike traps inside doors would swing down on the GI opening the door so they learned to hold their weapon in front of them to catch it. In respons, the trap were later buiöt in two parts, with a lower part that would then swing up and hit the family jewels. Ladyboy galore.
The guide was great, a really fun girl that knew a lot. Less fun was visiting a "handicapped handicraft" center where alleged victims of the aftermath of Agent Orange (the tree defoiliating dioxin used by US that can cause birth defects) were making the souvenirs that we have seen all over southeast asia. The ordinary cover for cheap tourist things but overpriced by crocodile tears.
All in all a very nice experience for a few hours (0830-1330, 11$, booked at hotel reception).