In the midst of an interesting post about historical versus pseudo-historical settings at Semper Initiativus Unum, Wayne includes a link to the International Catacomb Society, an organisation "dedicated to the preservation and documentation of the Roman catacombs [and] those rare vestiges of history that illustrate the common influences on Jewish, Christian, and Pagan iconography and funerary practices during the time of the Roman Empire." The site includes an interactive map of Rome showing the locations of various catacombs around the city in relation to other landmarks as well as floor plans of a small number of Roman catacombs.
Catacombs and other subterranean infrastructure, such as quarries and sewers and even human settlements, are 'dungeons' I don't feel funny about including in a historical roleplaying game campaign. They are wonderful settings for everything from covert entry to buried treasure, and I look forward to getting the characters in my campaign into someplace like the maze of passages beneath Paris - which won't exactly be catacombs for another couple of centuries - at some point in our campaign.
Who says the fantasy players should have all the dungeon-crawling fun, huh?
I added a link to the International Catacomb Society to the Ports o' Call collection of intreweb eclectica at the bottom of the right-hand column of RBE.