This is an underground tunnel Vietcong used to hide from American bombs and other weapons

"...but because no better alternative existed, the majority of the group in Saigon favored the attacks!" Robert McNamara, In Retrospect, regarding the bombing of North Vietnam. 

A New Style of War

     Unlike previous American victories against Japan and Germany, massive American bombing of enemy cities and use of heavy artillery would not be as effective.  The Viet Cong used a guerrilla, hit-and-run style of attack, which didn't allow the use of American artillery and bombs to kill them. The dense jungle fighting created low visibility from the air, which made it hard for the United States to utilize their far superior weapons. 
      In fact, America dropped seven million tons of bombs on North Vietnam and the neighboring countries of Laos and Cambodia (the bombing of Laos and Cambodia was outlawed by the Senate and done in secret.)  United States air forces only dropped 2.2 million tons of ordnance in the largest war in history; World War II. America dropped over three times more bombs, but it was still not effective enough to defeat North Vietnam, as well as costing the United States billions of dollars.
     The US strategy of attrition, destroying enemy troops and suplies faster than they can reproduce them, was not a viable stratergy. The Vietnamese who had been fighting for independence for thirty years were not about to give up.