I read about these Cu Chi tunnels and I thought they were very intruiging, especially in frustrating the Americans. Then I rembered that in the medieval period, mining under a castle wall and burning the wooden props to cause a part of a castle wall to collapse was a way to siege a castle, and the defenders could also dig their way into the besieging mines and attack the invaders.
Why didn't the US do this?
Also, I thought I rembered that in one of the Sieges of Vienna, Viennese bakers found that they could detect Ottoman miners with their water barrels and used them to find their tunnels to fend them off, and I've read that the Americans wanted to find where the Viet Cong soldiers were digging, so maybe some gadget that could pick up vibrations underground or something would've helped them out.
In 1966, the 25th Infantry Division built a large base inside the Iron Triangle northwest of Saigon. The base was inadvertently built on top of part of a vast enemy tunnel system, the Cu Chi Tunnels, which allowed the VC entry into the US base. After Operation Crimp, a series of battles fought in 1966 to locate and destroy the enemy headquarters in the Iron Triangle, and the discovery of part of the tunnel system under the Cu Chi base, existing US research into Tunnel Detection Systems was ordered to be deployed to South East Asia on a quick reaction basis. Research was being conducted into tunnel detection using Seismic and Electromagnetic detection systems to locate voids underground by ARPA (Advanced Research Projects Agency) from 1964, and this was taken over by USAMERDC (United States Army Mobility Equipment Research & Development Center) in 1966. The main problem to be addressed was the ability of the enemy to conduct military operations utilising bases consisting of elaborate systems of tunnels which were extremely difficult, if not impossible, to locate using existing detection systems. The systems were to find existing tunnel networks, and be easily transported and capable of rapid search. Research was fast tracked to find a suitable system. The seismic system used an acoustic echo pulse and a receiver. This system for detection proved too cumbersome for field use and was limited due to the extreme difficulty in interpreting target signatures. The Electro-magnetic detection system was more promising, with a man-portable system being developed. It was based on a pulse-echo radar system and tested at the the Engineer Proving Ground in Virginia USA. This was system was suspended when the program was cancelled in 1969, and was never used in Vietnam.
The US and Australians developed ways to detect tunnels and underground bases, mostly from observation of the area, notably trails that end at foliage clumps and gardens in remote areas, as well as odd odours and terrain features. They learned that tunnels could mostly only be dug in certain areas and at certain times of year due to the soil conditions.
The US and its allies did not use tunnel systems in Vietnam as they did not have an operational requirement to do so. VC and NVA tunnels were effective as a countermeasure to all but the heaviest US airpower and artillery, and served to frustrate infantry as they were well concealed and slowed down operational activity due to the efforts required to neutralise them. They were also useful as shelters for civilians during bombardment and as cache storage areas. The US benefitted from command of the air over the Iron Triangle and while US and Australian bases featured dug in and fortified areas, completely concealed underground bases were not a requirement. The US conducted research into electronic tunnel detection systems, but this was suspended in December 1969.
Sources:
Seismic and Electromagnetic Tunnel Detection Investigation by Henry F. Knauf United States Army Mobility Equipment Research & Development Center 1975
Vietcong and NVA Tunnels and Fortifications of the Vietnam War by Gordon Rottman.
Tunnel Rat in Vietnam by Gordon Rottman.
Vietnam Fire Bases 1965-73 by Randy E M Foster.
Why Were the Soil Tunnels of Cu Chi and Iron Triangle in Vietnam So Resilient? by Kenneth R. Olson, Lois Wright Morton 2017