The tunnel/bunker complexes encountered in the war zones were obviously the result of many years of labor, some in all probability having been initiated as early as WWII, with extension and improvement continuing throughout the war against the French and up until the time of their discovery by the US troops. These complexes  presented a formidable and dangerous obstacle to operations that had to be dealt with in a systematic, careful and professional manner. Additionally, they were an outstanding source of intelligence, as evidenced by the several tons of documents found during the clearing of the Saigon-Cholon-Gia Dinh headquarters complex in Operation Crimp, January 1966.

Tunnel Characteristics

The first characteristic of a tunnel complex is normally superb camouflage. Entrances and exits are concealed, bunkers are camouflaged and even within the tunnel complex itself, side tunnels are concealed, hidden trapdoors are prevalent, and dead-end tunnels are utilised to confuse the attacker. In many instances the first indication of a tunnel complex was fire received from a concealed bunker that might otherwise have gone undetected. Spoil from the tunnel system was normally distributed over a wide area.

Trapdoors were utilized extensively, both at entrances and exits and inside the tunnel complex itself, concealing side tunnels and intermediate sections of the main tunnel. In many cases a trapdoor would lead to a short change-of-direction or change-of-level tunnel, followed by a second trapdoor, a second change-of-direction and a third trapdoor opening again into the main tunnel.

Trapdoors were of several types; concrete covered by dirt, hard packed dirt reinforced by wire, or a 'basin' type consisting of a frame filled with dirt. This latter type was particularly difficult to locate in that probing would not reveal the presence of the trapdoor unless the outer frame was actually struck by the probe. Trapdoors covering entrances were generally a minimum of 100 meters apart. Booby traps were used extensively, both inside and outside entrance and exit trapdoors. Grenades were frequently placed in trees adjacent to the exit, with an activation wire to be pulled by a person underneath the trapdoor or by movement of the trapdoor itself. Typical trapdoor configurations are shown below;

Tunnel complexes found in the War Zones were generally more extensive and better constructed than those found in other areas. In some cases these complexes were multileveled, with storage and hiding rooms generally found on the lower levels. Entrance was often gained through concealed trapdoors and secondary tunnels. In the deeper complexes, foxholes were dug at intervals to provide water drainage. These were sometimes booby-trapped as well as containing punji-stakes for the unwary attacker.

Recognition of their cellular nature was important for understanding these tunnel complexes. Prisoner interrogation indicated that many tunnel complexes were interconnected, but the interconnecting tunnels, concealed by trapdoors or blocked by 3 to 4-feet of dirt, were known only to selected persons and were used only in emergencies. Indications also pointed to interconnections of some length, e.g. 5 to 7-kilometers, through which relatively large bodies of men could be transferred from one area to another, especially from one 'fighting' complex to another.

Bunker raised approximately one foot with one firing port