Tunnels Of Củ Chi, Ho Chi Minh City, Saigon, Vietnam

U.S. campaigns against the tunnels
The tunnels of Củ Chi did not go unnoticed by U.S. officials. They recognized the advantages that the Viet Cong 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 onto the region of Củ Chi, 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 looking for any clues of PLAF activity.
The operation did not bring about the desired success as for instance 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. 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, water or hot tar to force the Viet Cong soldiers into the open, or to toss a few grenades down the hole and “crimp” off the opening. The design of the tunnels along with the strategic use of trap doors and air filtration systems left the chosen approach to disable the Tunnel system completely, thus making them unusable thereafter, insufficient.