The Vietcong had been using tunnel systems for many years against the Chinese, Japanese and French invaders. There were numerous underground guerilla bases in South Vietnam that had been built and supplied by forced labor from the surrounding villages.
The largest systems were found in the so-called "Iron Triangle" and at Cu Chi and could be over 200 kilometers (130 miles) long. The tunnels would be constructed with water traps to keep out poisonous gases and be strong enough to withstand explosions and could house barracks, armories, supply depots, air raid shelters, classrooms, hospitals and factories.
The US employed specially trained men who volunteered to go into the systems in search of the enemy and information. They became known as "Tunnel Rats" and were hand picked men with nerves of steel, being selected for their particular ability to stay calm in highly tense situations. They could not suffer from claustrophobia and had to be of a strong but small stature. They would enter the tunnels armed with only hand grenades, a pistol and a flashlight, to do battle with the enemy. They frequently ended up in hand to hand combat and were subjected to snipers, booby traps, rats, snakes and scorpions. The Tunnel Rats received "hazardous pay" as extra compensation and would generally not serve for more than an average of four months.