If you dig down deep enough you'll find yourself in a place that's not really part of this world. Crystal caverns the size of cathedrals, shafts with no bottom, rivers that glow blood red. There's plants and animals your language don't have a name for. There's whole countries down there.
Agartha is the elemental plane of rock and stone. Alchemists call it Gnomon, after the gnomes, but most folks say Agartha. Outside of the Dreamlands, it's the plane ordinary folk enter into the most, completely unawares. Every deep mine has a tunnel that leads out of this world and into that one, and the older it is the more it has. Every miner has been to Agartha at least once in their life.
Prester John is the self-styled king of Agartha and founder of the Agarthan Scholomance. He's one of the Nine Wise Men, the graduating class of the original Scholomance back in Romania, and he's built himself a pretty little kingdom called Xanadu.
It's in a shallow pocket of Agartha under the Gobi Desert. The entrance is guarded from above by olgoi-khorkhoi, the famous Mongolian Death Worms. These are a kind of monster imbued with earthen energy to the point that they're as much mineral as they are meat. they can swim through stone and bore tunnels with their aqua regia vomit. Some of them have metal inclusions that can conduct electricity.
Xanadu is a single massive cavern, impossibly tall, supported by columns the size of skyscrapers. Prester John's magic keeps Xanadu lit as if with sunlight; fruits and flowers grow there year round. The buildings all look like the most delicate pagodas and stately temples. There are fields of tame subterranean deer; the people feed them grain by hand and harvest them for meat and milk. Massive colonies of cave-dwelling swiftlets live on the walls, and the people farm their nests for bird's nest soup. The people of Xanadu want for nothing. Except, sometimes, protection.
Like all the other planes, Agartha goes on forever, farther and farther down. And as I said, there's whole countries down there. The lost continent of Mu sank into Agartha, and Lemurian soldiers still patrol, monster-men grown from dragon's teeth, programmed to seek out enemies that haven't existed in ten thousand years.
There are enclaves of Faerie where dwarves, gnomes, and kobolds live. Their cities are grand and beautiful, built from the living rock, shaped through magic as to be all one solid piece.
Dwarves are stout folk, as you could imagine, with skin patterned like marble. Now, marble ain't just white or grey, it comes in a range of colors, from pink to gold to black to brown. Most dwarves I've met have a hint of blue to them, white hair like wool, and small, pointed ears, tufted with fur.
Gnomes are smaller folk, who love living with dwarves and among other races. They're famous for their little pointed hats; most people don't know that they're actually a horn made of some stoney substance.
Kobolds resemble goblins, but instead of looking like fruits and berries and such, they look like they were roughly carved from precious stones, and they usually have some horns on their heads as well. They're not as social as gnomes, and prefer to wander the tunnels, either alone or in family groups. Even still, you can find Kobold traders in every Agarthan settlement, even Xanadu. Usually, these guys work for the next group.
The djinni are the big merchants down there. Djinni can be born from any of the classical elements; the Deep Djinni as they call themselves have trade networks that reach all across Agartha and into other worlds. If you're ever out wandering the planes and find something ridiculous like a Walkman in a pre-industrial society, a Deep Djinni got it out there. For all that their appetites for money, good food, sex and power are very human, the Deep Djinni tend to present inhumanly. They often have a plain stoney exterior that cuts away to reveal rich glowing crystals and delicate organs of precious metal within. They make their clothes out of wire so fine it flows like silk.
Even deeper below them are the Gaolers. They were human, a long time ago, but the energies of the deep lands seeped into their bones, changed them. Places where the sun never shines are rife with curse energy, and it made them hunched, pale, hairless, with huge staring eyes to find their way in the dark. Almost every one of them is an Earth magus and almost all of their great practitioners are necromancers. Space is at a premium down there; they recycle their dead at every opportunity.
They don't kill prisoners though, not most of the time, hence the name (gaol means dungeon, you know). They know Prester John values life and will ransom back his citizens to him; denizens of the other lands aren't so lucky. The Gaoler economy runs on forced labor, dead and undead alike.
Between all these realms are all kinds of monsters, from the death worms to subterranean dragons. There are shoggoths here, and hidebehinds, cave elementals, forgotten ghosts of sunken cities, ghouls, and regular animals adapted to underground life, such as eyeless cavebears, titanic axolotls, deep swiftlets whose songs spell death...
But what's more dangerous than any inhuman race or monster are the tunnels of Agartha themselves. They ignore our rules of space and distance; they snake between worlds, stitching places together, and sometimes they shrink behind you, sometimes they lead you someplace they never used to go, sometimes a tunnel you've traveled a thousand times forks, and where that fork may lead could be a thousand miles from where you started, fathoms deep, in the path of a death-worm, a gaoler patrol, or something far, far worse.
