Sewers

While the dungeons of the fantasy worlds definitely are the classics for crawling around killing things and taking their stuff, what you might find in the sewers under a fantasy metropolis has been of interest to me.  I blame watching teenage mutant ninja turtles as a child.  A whole other world hidden beneath the feet of people who don't care about it, other than the fact that it makes their shit go away.  This of course brings up the possibility of shit based economics but no one really wants to go there.  An idea I heard of similar merit was a mechanical craftsman society living underground (example being dwarves) who tossed their broken and trash pieces of machinery and wizardry into a deep chasm at the edge of their territory.  At the bottom of said chasm live another society of creatures (goblins or some such) who have an entire economy (and possibly religion) based off the "gifts from above".  Food for thought. 
Art from here
LooneyDM out