with various tunnel under ground, they must take fresh air to breath!, see how the used this small hole as the ventilation point!.
But not sure, if this hole is the original holes used for the ventilation!.
The traps!, used to capture the animals,, well traps does not know if its is human or animals drop into it!.. can't imagine with those sharp steel piercing your body!
The trap door is carefully covered and camouflage
This is the way to closed the tunnel door!.
American tanks destroyed by the VC army.