Aztecs and a tunnel full of treasure:
Dungeons of Tenochtitlan (not the real article title)
What's interesting to me is that this is under a swamp-based city. Well, drained swamp based city.
The Aztecs are just interesting in a lot of ways - from their religion to their approach to warfare, to their end at the hands of Spanish cheerfully allying with all of the people the Aztecs stepped on to get to the top of the heap. Add in a dungeon and the DF campaign writes itself.
"The conquistadors departed, sealing the tunnel as they left, destroying the religious symbols. But the Aztec mummy-lich-wights in the tunnels below were not destroyed, as popular belief held. According to rumor, they're still there . . . and this map you found to the tunnel below indicates their treasure was not all recovered by Cortes and his 400 hirelings . . . "