The tunnels did not go completely unnoticed by U.S. officials. They recognized the advantages that the NLF held with the tunnels, and accordingly launched several major campaigns to search out and destroy the tunnel system. Among the most important of these were Operation Crimp and Operation Cedar Falls.
Operation Crimp began on January 7, 1966, with B-52 bombers dropping 30-ton loads of high explosive effectively turning the once lush jungle into a pockmarked moonscape. Eight thousand troops from the U.S. 1st Infantry Division, 173rd Airborne Brigade, and the 1st Battalion, Royal Australian Regiment combed the region.
The operation was, for the most part, unsuccessful. On the occasion when troops found a tunnel, they would often underestimate its size. Rarely would anyone be sent in to search the tunnels, as it was so hazardous. Besides being too small for most Western men to fit through, the tunnels were often rigged with explosive booby traps or punji stake pits. The two main responses in dealing with a tunnel opening were to flush the entrance with gas or water to force the guerillas into the open, or to toss a few grenades down the hole and “crimp” off the opening. The clever design of the tunnels along with the strategic use of trap doors and air filtration systems rendered American technology ineffective.
From :
http://en.wikipedia.org/wiki/Cu_Chi_Tunnels