There were approximately 300 miles of tunnels beneath Vietnam that were created during the French war of independence. The tunnels created a unique method of warfare. A Viet Cong could get out of a tunnel fire and then disappear back into the tunnel. They even had tunnels that had trap doors inside US compounds - so that they could jump out at night, shoot at will and then disappear!
In the words of Lieutenant Nguyen Thanh Linh:
"There were no set battles, but everyone who could fire a rifle did so. We used them for constant surprise sniper attacks, and we used them, most importantly, for observation. Thanks to the tunnels, we could remain with the Americans, see how their troops behaved and reacted, watch their mistakes."
The Americans went to war trained for set-piece confrontations, they were confused and demoralised by the way the enemy appeared and disappeared. This seems to be a classic example of 'single loop thinking'. They went prepared with one solution but the method of warfare was not prepared for the unique nature of the tunnels. If they looked back in their history it was guerilla warfare that they used to gain independence from the British. They had not thought deeply enough about the problem they were presented with.
As they discovered the tunnels, they had no idea how to fight against them. They used smoke to reveal trap-door exits but there were times when American lives were lost when the smoke they used suffocated the soldiers going down to fight the Viet Cong.