Tunnels of Mist

 Not every part-reality has a place physically. The Tunnels do. They're under. Under the earth, under the sea, under the mythic underworld. So far down the reality dissolves. Chunks of it are lost. Stone forgets it is stone, forgets its own texture and hardness. The Tunnels aren't filled with mist. They're made of mist, carved in it. Soft and light grey. Featureless, comforting. They welcome you and, if you let them, they'll aid you in your dissolution. The Tunnels can be glimpsed upon almost, but not quite, achieving ego death, or by cutting all your ties to the world and gently letting yourself fall through the cracks in reality (If your method is too violent, you'll fall into the Abyss instead, which is something barely anyone wants under most circumstances). Perhaps you could just dig long enough, but it is more likely your pick would forget its purpose and refuse to chip away at the stone before you got there.

The Land

Or whatever you wish to call "the part that isn't air". It's soft. It's gray. Swirly patterns emerge sometimes, but they are quickly subsumed into homogenous gray. It's not really mist. You wouldn't be able to walk on mist, obviously, but it's close enough, so that's what the people (the ones that still speak anyway) call it. The tunnels coursing through it usually vary in width from half a meter in diameter to several. There are some sections narrower than your shoulders, forcing you to squeeze through the tunnel like you're crawling through the artery of a decaying whale. The tunnels are rarely horizontal, nor does their layout makes much sense. Almost all sections slope to some degree. Some are almost entirely vertical. That rarely poses much of a problem.. The mist gives way, but it is not slippery, and this far down it is easy for gravity to forget it should pull you down at all.

From time to time the tunnels expand into a cavern. They are not a regular part of the landscape. Usually they are the result of the mist shying away froma new arrival in the Tunnels, still rich in identity and character, whether an object or a person. As features are drained from them the walls edge closer in, until the anomaly is absorbed and the area becomes a tunnel again. The layout is fluid. The same tunnels don't always lead to the same places and are, in fact, not usually recognizable. Cartography makes little sense for two reasons. Firstly because of , the aforementioned fluidity. Secondly, the featurelessness ensures there isn't much to map.

The... Features?

Until they are dissolved and their identity drained away, the Tunnels are the realm of things softly forgotten. You won't find a god outlawed and rebelled against here, though you might find one whose followers all quietly turned away from worship. This scrap-reality is rife with abandoned homes, forgotten imaginary friends, and dreams given up on. Or it would be, were it not in its nature to dissolve these things until they are no more, this time lost for good. This close to the edge of creation, the world's skin is frayed and thin. It is easy for bits and pieces of things to fall off and away, into the metaphysical void. Objects and ideas that fall down here start off vibrant and strong, some enough to force a bit of their identity on the surrounding mists. Slowly, surely, details drift away. They become blurry, gray, indistinct. In the end they always join the surrounding mist. 

The People

They are here, of course, but never for long. Some stumble in accidentally, others actively seek their own dissolution. A few wish to find and force back into hard full-reality that which has been lost. They rarely, if ever, succeed. They start out sharp, hyperreal against the featureless surroundings. The color leaves them first, then speech, then thought, then contours. Finally they are absorbed into the "walls" of surrounding mist. Some welcome it, seeing it as the ideal of ego death. Others are deprived of their will before they can protest. You can see them, sometimes, in their final stages. Wandering or standing in place like ghosts, except devoid of any kind of drive.

Deeper?

Maybe you could go deeper still. Before you went deep enough to perceive any kind of change, however, the reality around you would become too thin to support a "You" and so the transition would be meaningless. For practical purposes, this is where things end.