In an (apparently) infinite black region of nothing hangs a massive rope, about fifteen meters thick. It seems to go up and down forever in both directions, although gravity is normal, and runs in the direction of the rope. (So if you're in a random spot on the rope, you'd better hold onto it, or you'll fall.) In one part of the rope, scaffolds have been slung attached to the rope, allowing for a solid places to stand, arranged in many tiers. The largest of these are clustered together and between them support a population of a few hundred exiles, curiosity seekers, planar researchers, and some who were just born here and never left. Nobody knows what's at the top or bottom of the rope (if there is a top or bottom), or what's out there in any direction horizontally. The air is breathable but still, and there is no native life. It's also unknown what would happen if the rope were severed; it's unbelievably tough, but not at all impervious to damage.
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Most eyes look outward, to see what's outside of themselves. An eye that looked inside of itself wouldn't be very useful. This is vexatious to a peculiar aberration who finds itself in exactly this situation. The creature is little more than a giant eye, looking inward on itself. (Its organs, such as they are, are diffused across the outer surface.) The interior of the eye - which is all it can see, since it looks inward - is filled with a murky, salmon-colored translucent fluid, lit by light entering through the large "pupil". This interior is actually a demi-plane, where the creature transports people and objects for short bursts of time, in hopes of learning about worlds it can never see. The interior substrate isn't breathable, but creatures transported here enter a state of slowed animation, allowing them to be observed at length without necessarily drowning. (Though they sometimes drown anyway.)
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The mental planes are vast in number; it's not known if there's one for each person in existence somewhere or if they're called up when someone tries to access them, but there's certainly a lot of them. Each is keyed to a different individual, where an individual is defined as a creature capable of perceiving the world in some fashion. An individual's mental plane looks like a copy of the material plane, at least as they're familiar with it. Regions that they're intimately familiar with are more detailed, and regions they've never been to simply don't exist, or exist just as hazy emptiness. Additionally, mental planes are devoid of any other perceiving individuals (but still have plants and stuff.) Within an individual's mental plane, everything appears as the individual sees it, so a place the individual considers cozy and warm and has good memories of will make a positive impression, while a place the individual finds scary will seem that way to everyone who visits their mental plane. This property extends to visitors to the plane. If a plane's owner consideres someone especially handsome, weak, funny-looking, or whatever, that's the impression they'll make on everyone else visiting the plane (including themselves.) These are gut reactions; they don't reveal everything the mental plane's owner knows about somebody, just the things they feel inside when they see that person. (In some cases, this information can be pretty specific; for example, if a mental plane's owner's primary mental impression of somebody is that they're a murderer, that'll be what everybody who sees that person in the mental plane thinks, too.)
While it might seem like having your mental plane invaded is a severe violation of privacy (and it is), and thus you should avoid it at any costs, it's potentially more dangerous for the invader. Seeing things from the perspective of somebody else can rapidly warp your own mind. A wildly incompatible perspective can cause anything from mild dissonance to exteme shock; perhaps even scarier, seeing things from somebody else's perspective can simply, over time, change your point of view into theirs.