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
