There were about three levels if I remember rightly, one was larger and blocked up, that was the one with the story that it led to the cathedral, that I first heard in the mid sixties. As you say very unlikely (also any tunnel would have to negotiate the now culverted Afon Adda and the 'wet' ground of the valley bottom. My Taid remembered the original GPO building being built and said they had tremendous problems de-watering the foundation trenches). IIRC there was one behind
the terrace of houses on the foreshore, which was dammed and used as a water supply. The one you refer to, doesn't really go anywhere.
Here's a photo taken in 1982, as you can see nothing has substantially changed.
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