Cu Chi tunnels

The Cu Chi tunnels area bout 100km outside Saigon, and were a Viet Cong stronghold during the American war (as the locals call it to distinguish it from the many other wars they're had). The Viet Cong virtually lived underground here in 200 miles of connected tunnels. At their peak 16,000 people lived underground. They were used to launch surprise attacks on the Americans, who didn't figure out the significance or extent of the tunnels for a long time.

The Cu Chi area was peppered with B52 bombs, agent orange defolliant, bulldozers, psychological warfare, defections and spies. By the time the Americans pulled out they were on their last legs, but the tunnels allowed them to survive for much longer than a direct confrontation with a better armed enemy would have. I read a good book about it, so I can be very boring on the subject, but I might post in more detail later.