So as I am starting to flesh out a new world, the logical question came up: what, exactly, are the influences and works that have inspired this take?
There are literary influences, but most of them are safely "Appendix N" - Burroughs, Vance, Lovecraft, Dunsany, Merritt, Brackett, Norton - and if I listed some others a lot would be token. There's a bit of H.G. Wells (The Time Traveler, The First Men in the Moon) and some ostensible nonfiction like Chariots of the Gods? - but its not that interesting.
But that's all words. What really inspires me in terms of setting are look and feel. Perhaps because I've never been very good at art, I've always loved visual sources. When something strikes my eye, it makes a much deeper impression than a story.
As to why there's a city underground, well, it's because of this drawing.
It's a Holmes thing I guess - sticking that city under the dungeon is something I've wanted to do for a long time, an explorable high-level ruined metropolis.
Recently I saw Mike Mignola's concept art for Disney's Atlantis. This is what I want my ruined city to look like. I know the Disney city is supposed to be underwater, but think of it in a massive cavern instead. Those long staircases are simply epic. The towers have this ancient astronaut vibe to me, which I've covered before and want to use for some of the "looks" of ancient weapons and such. Mignola's characters also fit my vision of the blue-skinned underground humans, who I see having various tribal-esque tattoos. It's just such a different thing to see in a D&D game that I love it.
Another favorite of mine is Jean Giraud/Moebius's concept art for Alejandro Jodorowsky's Dune. Dune is maybe my favorite novel, and the mad vision of Jodorowsky may not be very close to the original, but people made absolutely stunning visual art for it. Moebius's characters have a visual flair that makes me want to create a world where they fit. There will definitely be suits of armor that resemble the Sardaukar here, and I really want to have people in my world who look like this.
Another big visual influence is feathered dinosaurs. Like a lot of kids, I was obsessed with dinosaurs when I was a kid. The modern vision is radically different. I love dromeosaurids, partly because "Velociraptors" were popularized by Jurassic Park, partly because the real things are also interesting. Besides, how much fun would it be to have turkey-sized raptors where most dungeons have kobolds? Scaled dinosaurs, of course, have a part to play as well; but the variety of colors and shapes is endless, and prevents things from getting repetitious.
Selenites (right) from an early French film of The First Men in the Moon are one of my favorite weird races, and conveniently they live underground, tending monstrous "mooncalves." Moreover, they are "specialized" based on their particular function in society. What a perfect alien race for science fantasy D&D! The French art makes them look perfectly bizarre while retaining a "humanoid" feel. I think with some variety, they could take up a good chunk of the role played by the goblin / hobgoblin / bugbear part of the cursus honorum of humanoids.
One of the things I really want to accomplish is a world that looks different from standard D&D. I really like the vision of how radically out of place the PCs will look in medieval gear, and the possibility that as the game goes on, they'll have more of a mixed aesthetic.
So that's the look, which is a lot of the inspiration.