Underground armament factory
The Viet Cong used to hide and funnel the smoke that came from their kitchen's, so that it would come out right next to a stream or river, the Americans would see the smoke and bomb, thinking the Viet Cong were living there. Some of the bombs would then drop in the water and not explode, the guerrillas would then retrieve the bombs, cut them up and use the explosives. Pretty clever!