They were there before the US base was built, they did have ventilation but there was no internet in 1965.
The US army couldn't work out how the VC were getting into the base, it took them a long time to realise where the insurgents were coming from, a long time.
And there's no way they didn't know.
It is physically impossible they didn't know, in fact.
But very much physically possible to claim they did not know with a straight face.
Perhaps it is possible that they knew but thought it was a different occupying force (perhaps “Hamas” is not what it seems to be)?
Edit:
I’ll just put this right here.
In the Vietnam conflict the US Army built a huge base on top of a giant complex of viet cong tunnels, there is a precedent.
Did those tunnels have office spaces with presumably internet and other infrastructure, with ventilation and air pumped in?
Were the tunnels there beforehand?
They were there before the US base was built, they did have ventilation but there was no internet in 1965.
The US army couldn't work out how the VC were getting into the base, it took them a long time to realise where the insurgents were coming from, a long time.