The Old Book Illustrations Scrapbook Blog has lots of images of interest to D&D players, and a few that scream "potential dungeon". Today, there were some pics from a book called Baksheesh! by Thomas W. Knox (1875). And two stand out:
The ruins of the Temple of Dendera, upper Egypt: Pillars! Ancient symbols! Even a team of heroes in the pic! Looking at this image, you can imagine a party of adventurers encountering this in the desert. They would have to climb down the drop-off in the front, which is probably at least a 20 foot drop, maybe even 30 feet. Somewhere in that enormous forest of pillars is an entrance to the actual vaults of the temple, where treasure and strange guardians still wait after millennia of silence.
The Serapeum of Memphis: Caverns! Twisty stone columns! A guy with a torch! It's not really the same location, technically, but easy to imagine as the subterranean vault hinted at above. A serapeum is a temple to the made-up deity Serapis, a combination of the dying god Osiris and the bull-god Apis. Made up by whom? The Hellenistic invaders, who wanted to unit the conquered Eqyptians behind their new order. That bit of actual history makes a good source of ideas for a backstory. It's something I don't think D&D directly addressed: can a fictional god become real? Although I suppose the Temple of the Frog sort of indirectly addresses this.