The main mystery of the Serapeum is, since the stone isn't local to where it wound up (IOW, it wasn't mined en situ), how did they get the finished boxes inside there?
Absolutely, and even more curious is how did they see to do it in the dark? There is absolutely no soot from torches or oil lamps on the ceiling.
I haven’t read much about the dimensions of the boxes vs. the dimensions of the hallway. I know there is one in the middle of one hall, left there without finishing up in a niche for some reason. Perhaps they made it too big—or just got tired of hammering, eh?