So, I like making up fantasy worlds, usually for my own enjoyment or as a setting for a fantasy story, but recently I started doing it for D&D settings. For 3.5e pretty much, as that's what I play.

Anyway, I've been working lately on converting my favorite setting to D&D. It's an Underworld setting. Not the Underdark; there are no drow, duergar, etc. here, and all the races live down there and have lived there for thousands of years since a World War III caliber conflict completely destroyed the surface world and made it irrevocably uninhabitable. Working title is Exodus, or Exodus to the Deep.

I don't consider it a "post-apocalyptic" setting because, even though technically there was an apocalypse, it was thousands of years ago and all the races have adapted to the Underworld to the point where it's not a struggle just to survive, and there are enough resources to sustain kingdoms on.

What I'm wondering is, does anyone know if there already is a d20 setting like this somewhere? I started writing this up as a simple homebrew setting for a campaign, but it's turning into a full-fledged Campaign Setting complete with enough info for a book a la Forgotten Realms Campaign Setting or Eberron Campaign Setting.

Off the top of my head, Dark Sun might give you a lot of ideas for the surface world at least.

You should definitely flesh out why there was an apocalyptic world war. Who was responsible, what were they fighting over, and how did they scorch/warp/otherwise make the surface uninhabitable?

Even if it's a mystery in-game, knowing why it happened can help you figure out what the surface is like, as well as how the underworld was settled.

My main thought is, do the inhabitants understand what had happened or is it just myths and lore? Do the groups of people blame other groups etc?

Yes, the inhabitants remember very well what happened, and have it well-documented in their history books. In the aftermath of the war after everyone migrated underground, the races made treaties governing the use of magic and other things to prevent another apocalypse, and also since their new home is more complex/fragile than the surface, they need to keep people in line to prevent some spellcaster from accidentally collapsing the whole place on top of them.

Still working on the details of the war and exactly how the surface was destroyed, but for a while all the cultures pointed fingers at each other, but after so many years those feelings have mostly died down. The real instigator of the war was a mysterious powerful race dwelling in the deepest caves that no one else is aware of, or if anyone is aware of it they're in a minority and aren't believed. (Kinda like "the Others" in A Song of Ice and Fire, but even more secretive; these "others" are so unknown they don't even have a small place in the mythology or legends of the culturess.)

So it will feel a little similar to Song of Ice and Fire, or the Midnight campaign setting, in that there is a powerful mysterious overarching "great enemy" to the setting. Except these enemies don't make themselves known because at least their plan has already succeeded. Still fleshing out these "enemies" and why they forced everyone to move underground. Perhaps these enemies are a cursed race who were punished by the gods to never see sunlight again for some archaic misdeed, and now they want everyone else to suffer similarly, or something along those lines. Not sure yet.

Yeah Morph3us, I will definitely look into Dark Sun for ideas of the surface. Except in this setting it's not even physically possible for people to live up there, since the air and water are toxic (think magical version of radiation perhaps). Of course, undead and demons and such like it just fine. I haven't given much thought to an actual layout of the surface just yet, as the main focus of the setting is the Underworld.

One big hurdle I have though is maps. It's easy enough to map out the caves from a top-down perspective, but caves are in 3 dimensions. Not sure how I could represent that in a 2d way. Modern speleologists have sophisicated 3d mapping software for mapping cave systems, but I'm not sure how people in a fantasy setting would accomplish that. This setting is separated into 3 layers: Shallow Lands (humans, halflings, orcs, various half-breeds, a couple new races), Krogaunud (dwarven for 'twisted tunnels') aka The Honeycomb, a no-man's land of bewildering twisting tunnels with few resources, and the Deep Lands (elves, dwarves, gnomes, various half-breeds, a couple new races). Hard figuring out how to represent 3 dimensions. I gave up on any notion of making a map of The Honeycomb since its very nature is too bewildering to map...

Well, the same way contour maps are used for 3D terrain on the surface, cave with their "tops" removed could be mapped the same way. They would probably have to use separate maps for "layers" in places where the caves are stacked over each other.

I'm using such a method in an adventure I'm running right now, which is just a brief underground excursion.

Minor off-topic note; joyously stolen from a friend of mine who builds more worlds and gaming systems than... well... Monte Cook:

The poisonous water and earth is not completely uninhabitable. Some race -pick one, it matters not- has somehow grown adapted to the poisonous water and crops. So much so that they cannot live without it and thus would need quite a bit of their own supplies if they decided to go underground. This makes their kind much less adventurous into the underground because they do not survive without their own 'dose' of food and water.

However, this idea was put into place in his world as a mechanism of roleplay and culture. I'm not sure it would truly work in a world so completely under the dichotomy of overland vs underworld. In his world, those few people that needed the poisonous fruit were forced into ghettos in every major city because it was there that they could bring their water and it was there that their crops could grow. One city, the largest and focus of his playtest, had a sewer system and was constantly on watch to make sure the poisonous water never entered the groundwater sources. The ghetto was an overzealous prison more or less. The player that played the 'poisoned-raced' had to be very careful on choice of adventuring and quests, but she made for a great roleplaying opportunity AND was quite at home when the plot took them into her restricted zone.

Yeah, that wouldn't work for me. In my setting the surface might as well be the vacuum of outer space. This setting = Underworld, period. The elves tried their hardest to magically make the surface livable, but even they failed and were forced underground...it was baaad. Even brief trips to the surface are ill-advised. And when I mean toxic, I mean like, "acid gas eating away your skin within moments" toxic. Not like "trace amounts of sulfuric acid" toxic. lol

Basically, the very point of this setting is that it is Underworld...not even the remotest possibility of living/adventuring on the surface, except in dire circumstances. The surface is on a level of hostility to prime-material natives as the Elemental Plane of Fire or the Negative Energy Plane. Not going to survive for long without incredibly potent magical/supernatural defenses.

KillerK... I <3 your settings.

Now, you just KNOW the players are going to want to see what's up there at some point. And its toxicity means there's a good chance there could be some interesting and untouched ruins+treasure.

Again, I'd definitely look at mapping in layers for the underworld. Also, a side view map might help the whole thing make sense together. Kinda like Expedition to Undermountain did for the titular dungeon.

Sounds vaguely like the 'Hollow Earth' setting, with the addition of the apocalypse and various other details.