http://www.diggerhistory.info/pages-conflicts-periods/vietnam/tunnels.htm
http://www.showcaves.com/english/vn/misc/CuChi.html
Củ Chi Tunnels are a good example of why the U.S. Army had little success. They were built to fit the small Viet Cong guerillias, had lots of traps and were perfectly hidden. The soldiers left the tunnels, hit, and then vanished without a trace. The tunnels were built by hand, without any complicated or expensive tools, and thus there were little needs for supply, except food and weapons. They were used as communication and supply routes, hospitals, food and weapon caches, and living quarters for numerous guerrilla fighters.
The tunnel system originally stretched all the way from Saigon to the Ho Chi Minh Trail at the Cambodian border. Its total length is given with 250 kilometers. Obviously this was not built in a few years, it was started during the war for independence from the French, and its construction was continued for 25 years. People lived in this underground city for years, there were kitchens, living rooms, even schools and theaters. Up to 40,000 Vietnamese people perished in the tunnels.
The construction of the tunnels was only possible because of the special geology. The area around Saigon is covered by a thick layer of red clay, the residual of tropic erosion. This clay is easier to mine than hard rock, but its solid enough not to collapse. It was dug with hand tools, reed baskets filled with clay and then dumped into bomb craters. There were vents for all puroses, fresh air, chimneys for heating and cooking fires, even for hearing approaching helicopters.
It was a joke of destiny, that the U.S. Army built the headquarters of the U.S. 25th Infantry Division right on top of the tunnels. This made them ideal for any kind of sabotage. The army did not realize their existance for some time, but when they did they had no possibility to destroy them. The huge American soldiers did not fit into the narrow passages, additionaly they were dark and filled with deadly booby traps. But bombing the area did neither work, the soft clay damped the shock waves, the tunnels were unharmed, or if some collapsed they were replaced immediately. "The most bombed, shelled, gassed, defoliated and generally devastated area in the history of warfare" (Tom Mangold and John Penycate) was probably the most important reason, why this war was lost.
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