There are nearly endless varieties of caverns, tunnels, settlements, and ruins. The underworld as a whole is divided into two main areas. The first is the innerworld, which common knowledge holds is the very center of the planet. It is the closest that the people who live down here have gotten to being outside. Big enough to have a proper sky. It has weather, a entire ocean, and even a sun. A bright star that shine with true sunlight in the middle of the innerworld. It always shines, there is no night or day. Only the constant bright light of the inner star. Which is why most people stay in the underworld proper. Such huge open areas make most underworld denizens uncomfortable and the constant never changing sunlight tends to drive people crazy after a few weeks unless they grew up there. Plenty of small settlements, but the only major power in the inner world is the Ivory Dominion that rules over the great inner world ocean.
The other is the series of caverns and tunnels that seem to be wrapped around the inner world. This is the underworld proper and is where most denizens live and die. Where the biggest cavern is one you can still see the ceiling and the only light is from bio-glow fungus and magic crystals. Past a certain point, more or less a few dozen miles away from the inner world, the caverns get even more dangerous. The magical comforts (such as they are) vanish and only the true inhabitants of the underworld can live. Many scheme to evict the surface dweller trespassers so the caverns will be dark and quiet once again.
Basically, the innerworld is ‘outside’ with weather and semi-normal animals and more or less ‘normal’. Which any underworld inhabitant will find extremely weird and disconcerting. The underworld proper is the main setting where most adventures and travel take place. The deeper darkness (ironically those parts are technically closer to the surface of the world) is the setting’s Underdark equivalent. Where the incredibly dangerous creatures and surreal encounters happen. Even hardened adventurers make a point of not going too far from the underworld caverns that generations of magic and refugees have made more or less livable.
Oh yeah, there is also the Abyss. A portion of caverns that *nobody* goes to willingly. Don’t go to the abyss. Even if you survive, you won’t find your way out. They couldn’t find any better name then the plane of pure cruelty and chaos and evil. Don’t. Go. To. The. Abyss.
Maps in the underworld are tricky. The innerworld can be mapped in a way a surface-dweller would recognize. The more three-dimensional nature of the underworld proper makes large scale maps nearly impossible. Not to mention that landmarks aren’t something you can see in the distance. So maps are less precise drawings of a location and more coded directions. A map has to be identified with points of reference before it can be properly matched to a location and used.
Example: A treasure map might have a decent floor plan of a ruined castle, but to get there you would have to match the reference points around the castle to find it. Such as: A vein of magic sapphire, an abandoned demon shrine, and a gigantic wrecked boat.
Most general maps use fairly well known points of reference, often entrances to the innerworld, or well-known locations. More obscure maps might use precise reference points that only a handful of ancient elves might know about.