This is boring, but I work in Water Utilities and found the water network at Pompeii fascinating. I'd already got used to the idea of flushing toilets and drinking fountains in medieval castles, but this was even more advanced.

On a similar theme, Mary Beards book on Pompeii goes deep into the minutiae and makes on the surface boring topics fascinating. For example her analysis of Pompeii's one way traffic system.