The tunnel system, built over 25 years starting in the 1940s, let the Viet Minh and, later, the Viet Cong, control a huge rural area. It was an underground city with living areas, kitchens, storage, weapons factories, field hospitals, command centres. In places, it was several stories deep and housed up to 10,000 people who virtually lived underground for years.... getting married, giving birth, going to school. They only came out at night to furtively tend their crops.
The ground here is hard clay, which made this whole thing possible. But even so, the planning and construction was incredible. People dug all this with hand tools, filling reed baskets and dumping the dirt into bomb craters. They installed large vents so they could hear approaching helicopters, smaller vents for air and baffled vents to dissipate cooking smoke. There were also hidden trap doors and gruesomely effective bamboo-stake booby traps.
Of course, the U.S. military knew about the tunnels. The tunnels not only allowed guerrilla communication, they allowed surprise attacks, even within the perimeters of U.S. military bases. The U.S. retaliated with bombs, eventually turning the region into what writers Tom Mangold and John Penycate called "the most bombed, shelled, gassed, defoliated and generally devastated area in the history of warfare."
Opening to a bunker system, found by Australian soldiers on Operation Coburg.
