Late August, AD 79, a few days before the eruption of Vesuvius: Aquaduct engineer Marcus Attilius Primus comes to the Bay of Neapolis to take over the maintainance of the Aqua Augusta. Not only does he have a mysterious breakage in the aquaduct to deal with, but he's also got to work out what happened to his predecessor.
The intricately-detailed setting meant I enjoyed this rather more than "Enigma" or "Archangel". My only complaint was that the ending felt a bit contrived -- although it could have been much worse, and the reader's sense of impending doom limits what can be done with the plot! Recommended, anyway, particularly if you find engineering interesting.