Job #1 Submission - The Abyss.
It's been a little difficult for me to find images to explain this, but allow me to explain.
Essentially, the idea for this is that the planet that this "world" is set on, well, the surface is uninhabitable. It's essentially a desert wasteland, and very few people even reside there given the exceptionally toxic air. The actual "world" is far underground. See, in this world, there's essentially a "cave" that's like, around 10km below the ground, but is around half the size of the planet, and is around 500km deep. The "cities" here are very much a steampunk sort of style, but all of them are hanging from the crust at the very top, given that it's bedrock above them. Because of that, cities are very much built top-down - as opposed to the traditional style of "the most wealthy are at the top," cities are built like inverse pyramids with the upper layers being for the poorer folk whilst the lower layers are where richer people reside.
There's actually a couple of cities like this, as well as a vast amount of small "villages" which are essentially built from scrap materials, hanging from the ceiling - and all of these are connected by a vast, interconnected amount of bridges, through which trains will travel. Whilst airships are occasionally used, it's rare to see them, mostly because of the sheer amount of gas on the upper layers. However, this makes it intensely dangerous to travel between cities and the like, and thus it's quite common to see "pirate trains" on the Rails, as well as nomadic groups who take the Rails as their home, and travelling merchants who move between cities in order to do business. The Rails are quite well known for being on several different layers - it's a common practice for Rail Pirates to hog upper Rails in order to leap down to lower ones in order to pilfer their goods, and then use winches to get themselves back up.
In terms of the biome itself, the bottom of the cavern - being around 500km deep - is essentially a death sentence, and nearly impossible to physically reach. The bottom itself is usually where waste is tossed - with the actual "floor" of the cave consisting of a vast amount of incredibly thick, bubbling oil which is heated by the core of the planet itself. Notably, the upper levels of the cave are filled with a variety of fumes, most notably large amounts of toxic and extremely flammable gas which make it nearly impossible to breathe - meaning apparatus is required to effectively operate airships in the Abyss. It also means it's quite possible for large explosions to occur - for instance, one of the more notable cities there exploded violently after an airship crashed into the ceiling of the Abyss, falling downward and killing thousands of people.
I also like to imagine that light is considered to be a premium, given that people live in this vast cavern. Coal is the main means of energy, but given that it has to be dug from the ceiling, it's intensely dangerous to do so - with mining being essentially the most dangerous job in the entire system, but being mandatory service for people (a little like how Switzerland has "public service" as mandatory for a certain period of time.) As such, having light is rare - and thus, having more light in your establishment is considered to be a sign of wealth or esteem. There's also a particular ore, known as Lumina, which constantly gives off a soft green light - originally thought to be a wonderful thing, but eventually found to be exceptionally toxic. As a result, Lumina is often used by miners in place of fire due to the sheer amount of gas on the upper levels of the Abyss.
The cities themselves are built in a particularly ramshackle manner - having been built from the top down, they are essentially a mess of exposed walkways, buildings and huge cables holding the cities in place - as well as a series of train tracks that pierce directly through the cities in order to most effectively deliver stuff.
Anyways, this is just my own basic idea - hope y'all like it!
