In 1965, soon after the first US combat troops had arrived in Vietnam, it was realized that in some areas the Viet Cong had developed vast tunnel complexes in which to hide from the enemy. It was long known that such complexes existed, but it was not realized just how extensive they were in some areas, how important they were to the Viet Cong, and how difficult it was to detect and neutralize them. At first infantrymen volunteered to enter the tunnels armed with only pistols and flashlights the ’tunnel runners’ were born, known to the Australians as ’tunnel ferrets’. Starting as an ad hoc force of infantrymen, combat engineers and chemical troops, it was not long before units were ‘formalized’ as ’tunnel exploration personnel’ and 4 6-man ’tunnel exploitation and denial teams’ were created. They came to be known simply as ’tunnel rats’ with the unofficial motto Non Gratum Anus Rodentum ‘Not Worth a Rat’s Ass’. This title will be based on the personal accounts of those who served in this unique role and will describe the specialist training and equipment, not to mention the tactics and combat experiences, of those who fought an underground war against the Viet Cong in Vietnam.