No wonder we couldn't find those guys. A couple of divisions could hide in a cave this size! It's Son Doong cave in Phong Nha-Ke Bang National Park which is about 35 miles north of the DMZ and east of the Ho Chi Minh trail (out of our range).
“They see only their own shadows or the shadows of one another, which the fire throws on the opposite wall of the cave” -- Plato
“If there hadn't been women we'd still be squatting in a cave eating raw meat, because we made civilization in order to impress our girlfriends.” -- Orson Welles