You lower yourself down into the 'spider hole', that tight hole in the hard red soil that’s barely wider than your shoulders. You are stripped to the waist, and you have an Army-issue flashlight in your left hand and a .45 caliber M1911A pistol in your right.

Your body is shiny with sweat, but not entirely from the scorching heat of the tropical sun and jungle humidity of [Vietnam]; you are more afraid than any time before in your life, more afraid than in any firefight or even during that first night patrol, because now you are facing all of the worst fears together: the fear of impending death; the fear of the hostile, black unknown; and the unique fear of crawling into your own narrow, coffinless tomb. You are about to elbow-and-toe your way through narrow, hot, dirty, poorly-ventilated tunnels. You are following [Charlie] into his dangerous lair, where he waits, ready for your intrusion.

When you drop down, will you fall into a [punji pit] or be shot immediately by a guard from a hidden firing position? As you struggle along in darkness (you don’t use the flashlight until you need to) will you fail to notice a booby-trap trigger and end your life's story then and there? Will you drag yourself over a nest of fire ants that will introduce you to hell with a thousand stings? Or maybe a swarm of crab spiders? Will you squeeze head and shoulders through a trap door, only to be strangled or have your throat cut and wait helplessly to die? Or will you have to be dragged out, feet first, screaming and crying because you finally just lost it from the pressure and fear? Any of that can happen to you, as it already has happened to others. You are a tunnel rat.

The term tunnel rat is Vietnam [war|conflict] era army jargon used to refer to soldiers who crawled into tunnels dug by the Viet Cong (VC, Victor Charlie, or just Charlie) to search for enemy personnel or intelligence before the tunnel was destroyed with explosives. At first these men were called ‘tunnel runners’ or ‘ferrets’ (a very apt term used by the Aussies), but the more colorful label ‘tunnel rat’ soon became standard.

The US Army first learned of the extensive use of underground tunnels by the Viet Cong during [Operation Crimp] in January [1966]. Operation Crimp was a large-scale joint operation that involved parts of the 1st US Infantry Division, the 173d US Airborne Brigade, and the Royal Australian Regiment in a search-and-destroy sweep through an area near Saigon that had been under VC control for a long time. The 1st Infantry Division was suffering casualties right after hitting the [LZ], but as they and the other units pushed on, the resistance faded. Nevertheless, casuaties from snipers continued to mount. The sweep ended at the Saigon River, where it was expected that the enemy would be trapped, but no masses of enemy troops were found. They had apparently vanished.

The mystery was broken when the Airborne Brigade and the Aussie regiment found the VC in an underground complex. The 1st Battalion then went back over the ground covered during the sweep, looking for tunnel entrances, but none were found. The breakthrough finally came when a soldier was literally bitten on the ass by one. Platoon Sergeant Stewart L. Green felt a sharp pain when he sat down for a rest. He leaped up and poked through the grass and leaves with the muzzle of his M16 for what had bitten him. It could have been any of several poisonous denizens of the jungle bush. What he found was a nail sticking up from what turned out to be a hidden trap door to a tunnel.

Sergeant Green became the first tunnel rat when he volunteered to go down into the tunnel he had discovered. At 130 pounds, he was well-suited for the nearly shoebox-sized entrance and the claustrophobia-inducing tunnels. He was joined by several of his brothers. They discovered and retrieved a lot of equipment and supplies, as well as a group of about 30 VC, which prompted the rats to make a fast retreat. Efforts to flush the enemy out with smoke and riot gas failed and the unit had to hurry on with their crucial mission of covering another unit's flank, so the order was given to place demolition charges in the tunnel and it was destroyed.The underground facilities ranged from simple dug-out rooms for caching food, weapons, munitions or medical supplies to large, interconnected systems that had kitchens, rooms for sleeping, storage, medical treatment, and political classes. Some were connected to bunkers, fighting trenches and anti-aircraft positions. They were not generally used by regular NVA troops or even by the mobile, organized VC fighting units, but were most often used for local defense and air-raid shelter by VC villages and training by political [cadres] or as a refuge for command centers. It was quickly learned that they often held documents that provided extremely valuable [intelligence], so it was essential to explore them before destroying them to deny future use.

The huge complex of tunnels at Cu Chi, just northwest of [Saigon] (now [Ho Chi Minh City]) is a popular tourist attraction today. As the war went on, more and more tunnels were discovered, and the demand for tunnel rats increased. Lessons were learned and units of tunnel rat specialists were organized and the motto, "Not worth a rat's ass" was adopted. (creds to [a] for this bit.)The tunnel rat had to be small, brave and very even-tempered to handle the stress. The .45 was replaced with a smaller, lighter .38 caliber pistol, often equipped with a silencer. The .45 was heavy and its loud report was truly deafening in tight quarters and firing it often left the tunnel rat unable to hear the enemy. Other gear included insect repellant, a compass, a probe for locating mines and trip devices, a combat knife and a wired communication device to keep topside informed and to allow crucial mapping as the tunnel was explored. The tunnel rat's job was not always as intense as depicted in the first paragraph above, particularly after some degree of expertise had developed through experience. Some found the job exhilarating and a welcome change of pace from normal infantryman duty. Nevertheless, there is no doubt that these volunteers were exceptional men who clearly went beyond the call of duty to do a highly dangerous but highly valuable job.

http://www.soft.net.uk/entrinet/tactics
http://www.army.mil/cmh-pg/books/Vietnam/907/ch6.htm
http://wire.ap.org/APpackages/20thcentury/67vietnam.html
http://www.virtualtourist.com/m/.56745/article/96/10/?s=S

Tunnel rat (person)