UPDATE: Here's U.S. Army training video featuring some tactics against tunnels.
So, a special salute to the "tunnel rats" on this Veterans Day 2015:
During the Vietnam War "tunnel rat" became a more or less official specialty for volunteer infantrymen, primarily from the United States, Australia and New Zealand. Their motto was the Latin phrase "Non Gratum Anus Rodentum"—"not worth a rat's ass". Since the 1940s, during the war against the French colonial forces, the Viet Cong had created an extensive underground system of complexes. By the 1960s, there were underground hospitals, training grounds, storage facilities, headquarters and more. The Viet Cong, who were crack forces highly skilled at guerrilla warfare, might stay underground for several months at a time. The tunnels were their territory.Brave men.
Whenever troops would uncover a tunnel, tunnel rats were sent in to kill any hiding enemy soldiers and to plant explosives to destroy the tunnels. A tunnel rat was equipped with only a standard issue .45 caliber pistol, a bayonet and a flashlight, although most tunnel rats were allowed to choose another pistol with which to arm themselves. The tunnels were very dangerous, with numerous booby traps and enemies lying in wait.
Tunnel rats were generally, but not exclusively, men of smaller stature (5'6" and under) in order to fit in the narrow tunnels. Mangold and Penycate claimed that the tunnel rats were almost exclusively White or Hispanic soldiers and that the majority of American Latinos were Puerto Rican or Mexican American. Such tactics came to prominence following their successful application in January 1966 during a combined US–Australian action against the Củ Chi tunnels in Bình Dương Province, known as Operation Crimp.