If you've spent some time around underfolk, you know well that there is another world of sorts below the surface.
A world of darkness broken in places by small spots of bioluminescence, a world of both wide caverns and tight crevices, great riches and terrible dangers.
But if you've paid attention, you may have realized that there is no real end to this underdark.
Caverns themselves often have ends, but cavern systems do not: there are always a few paths which remained unexplored.
So what is there that lies further below?
There are multiple conflicting stories. Some will say that deep below all that rock lies and ocean, and that the world itself is an island.
Some will say that the rock gets warmer until it is burning like embers or flowing like molten iron, the home of demons and dragons alike.
A few will even tell you that below our world is another, identical to our own but with everything upside down.
All these ideas are various degrees of ridiculous, but we have no way to prove any of them right or wrong.
But no matter what lays at the bottom, we have more credible stories about what lies in between. Credible but far from banal.
The underfolk call it simply: "The Mist".
The Mist is much like our own world, except that it has a roof of stone instead of an invisible firmament. But in the gigantic and foggy caverns of this strange place, it is easy to forget you are imprisoned by stone: when surrounded by cliffs, rocky islands and immense pillars that stretch both up and down into the nothingness, with no floor or roof visible, it is hard to keep yourself convinced that this is not a world of its own.
And why wouldn't it? Just like ours, this world is teeming with life, but it is strange, remote from anything on the surface:
vines of deep green stretch towards the horizon in lazy curves, dotted by small pods of orange-yellow light. Plants that seem to float freely in the air, like lanterns lighting an invisible aerial path. Grass that curls at the tip and emits large spores similar to mushrooms', which cloud nearby air and contribute to the already dense fog. Strange creature which act like bats and birds yet look more like crabs upon closer inspection, and fireflies larger than your hand.
The Mist is a quiet place, almost to an unsettling degree. Only the cold breeze and the creaking of vines keep the heavy silence some company.
A single step can be heard many paces away, but who is there to hear it? Hopefully, no one. It is not fully known what lurks in these depths, nor do we want to know. Few venture down there and few come back entirely: insanity is common among those who tell these tales, but different stories from different mouths align too much to be entirely discarded as the ravings of madmen.