My historical research took a wander underground recently, partly inspired by the Canynge Square sinkhole, partly by St Vincent's (Ghyston's) cave and its tunnel to the Observatory, and I was surprised to find that there might be an intact tunnel from the Bristol Port Railway and Pier still just sitting there under Bridge Valley Road. A quick search turned up this recent video by an intrepid explorer, so it's definitely still there.
I went looking for the entrances today, and definitely found the south entrance, at the start of the Bridge Valley Path, the footpath that starts with steps at the bottom of Bridge Valley Road. It's easy to miss if you're not looking for it. I think I've figured out where the north entrance is, too, but it was getting dark at that stage and the Portway was still busy enough that crossing the road was still the normal nuisance, so I thought I'd leave further explorations for another day.