But calling it a space, or any other thing actually, will do. We must call it something in order to talk about it. It is a distinct weakness of our language that it has no words to describe the indescribable. Therefore all these words chosen to do so should be viewed only as imperfect metaphors at best.
Different cultures have named this thing differently and imagined it with very different properties. It has been named the astral sea, the beyond, chaos, the essence, the flow, the in-between, the magium, the mists, the nothing, the outside, the primordial chaos, the strange, the unreality, the unspace, the void, etc.
Position in the Omniumverse
While the Realium contains everything that is real, the Imaginarium contains everything that is not strictly real. This includes things that are just not real but also things that are both real and not real at the same time as well as things that are neither real nor not real or even things for which those properties don't even apply. Things that aren't even things, paradoxes, etc.
What is it?
One could say that the Imaginarium is unreality, a sea of Imaethyr, the element of which imagined things are made of, before being molded and given form by the process of imagining them.
Whenever anyone talks about the Imaginarium, it is assumed that they are talking about what could be called the Shallow Imaginarium, not the entirety of the Imaginarium. Unless otherwise stated, everything in this text applies to the Shallow Imaginarium specifically.
Its nature is that it has no true or real nature.
It has no laws of any kind governing it, physics or otherwise, and no physical substance at all. It is subjective in its essence, without any stability unless what ever is imposed upon it. It has no space and no time, no dimensions of any kind. It has no form or shape for a shape presumes some kind of space. The Imaginarium is a non-spacial space where exists all that "is not". It is not a space and it has no space but it can have many areas on which space exists, in one way or another. It doesn't have time either, but time can exist in many ways in many different areas inside it.
Attempts at defining it
It cannot be defined except in this weak way through nonsensical analogies and indirect references. It is not something that can be defined either by language, mathematics or logic of any kind. It transcends all those concepts, it transcends concepts and conceptualization itself. It includes all those concepts as well as all else that transcend them. Things don't have to make sense inside the Imaginarium and probably don't most of the time.
Whatever it is imagined or believed to be, its reality is more than that. Even indirect and infinitely recursive definitions or references are not enough to capture it. Nor is any other linguistic, logical or imaginary mechanism to do so. It's ultimate definition is that which cannot be defined.
Even the previous definition doesn't make it any justice though for it is beyond what can be imagined by anyone, real or imagined. It itself cannot be fully imagined in its entirety for any and all attempts just result in the stabilization of an area inside being defined in the way imagined.
Imaginarium is just a word that is being used to refer to it in a necessarily unsatisfactory manner.
It cannot be said to have a overall structure. Despite that it is convenient to understand it as having different areas inside it, each with different properties. It is important to remember that this is just a metaphorical way of trying to understand it though, not its actual reality for it has no overall structure, except where it has one.
An area can be as large as it needs to be, from negative sizes to infinite sizes, transfinite, impossible size, no size at all, all of the above, none of the above or both at the same time. Also, each area can be subdivided into more areas inside it that can share some of the properties of the larger one or none at all. Each area can be infinitely divided in more areas and those in more areas and so on.
The areas inside can also contain the area containing them, can be larger than the area contained them, etc. Paradoxical definitions and impossible relations are not uncommon. This is how the Imaginarium contains itself infinitely many times for any of its areas can easily contain the whole of it or even more.
The Shallow Imaginarium is effectively one of these areas. The area we are most familiar with. The Deep Imaginarium is all the other areas along with everything not in the Shallow Imaginarium.
There isn't necessarily a noticeable change between one area and the next. It can be so gradual to the point of being unnoticeable by anyone but it can also be very sharp and drastic. What this means is that these areas aren't necessarily concrete things, they can be subjective, inconstant, mutable and unstable.
Deep ImaginariumIt is convenient to to imagine the Imaginarium as being divided in two big areas, the Shallow Imaginarium and the Deep Imaginarium. This makes it easier to talk and think about it, however it can't be said to fully correspond to its reality. Even then it is a useful analogy.
The Imaginatas existing in a given area and even the properties of that area are all derived from the imagination of the inhabitants of some other Imaginata located prominently on that area as well. The larger area encompassing them all forms its Shallow Imaginarium. This is true for the Realium as is for any other Imaginata. This means that the term Shallow Imaginarium is relative from one's point of view or origin.
So the Shallow Imaginarium is the area containing all the Imaginatas that are directly derived from the imagination of those inside the Realium. After that, a larger area contains all Imaginatas directly derived from the imagination of those inside any of the Imaginatas of the first line. This area already can be said to be a little less shallow, a little deeper, whatever that means. It is a little less familiar, a little stranger. After that comes another larger area with all the Imaginatas derived from the imagination of the inhabitants of all the Imaginatas of the second line and so on for all the infinite chains of derived Imaginatas from derived Imaginatas. Some of the times these areas overlap with Imaginatas being simultaneously derived from the imaginations of multiple other Imaginatas. Sometimes even loops of imagination are formed, from pairs of Imaginatas each imagining the other or even more complex loops.
Only after all of that does one get to the real Deep Imaginarium. First it only appears as a place where infinitely many other unrelated chains like that exist but even deeper than that is where no such chains exist. Where no Imaginatas exist. Where other things, different things exist. Where it is not permeable to imagination in the same ways or at all. Where it can be described in different ways, maybe described like The Box, maybe in other more different ways. Where all the text on this page ceases to apply. Where the non-imagined and the unimaginable things exist and even beyond that.
Relations with other Cosmological Objects
The realms that god-like entities rule over have been called Godverses. They are usually attached to one or more Imaginatas that their rulers control or have even created. They are different from normal Imaginatas in the sense that they are much more connected to the existence of their own rulers and are kept stable by their incredible mental powers alone.
The areas between the clusters of Imaginatas are much more empty, as the focus of imaginations usually are concentrated together to some extent. Those empty areas correspond to the Outside. Even further away, beyond those areas is the Beyond. Areas not filled with the imaginations, not empty of them but instead filled with their opposite, filled with 'amaginations' or 'unmaginations', whatever those are. But those areas are extremely far away, across many infinities of non-space. They exist beyond even the Deep Imaginarium.
Paradox Space is probably the equivalent of the Imaginarium if not exactly the same thing, with their descriptions being closer than anything else. More likely it just corresponds to the Deep Imaginarium instead. Both contain all that is not real and both are filled with paradoxes, even being paradoxal in nature.
The Box can also be seen as the equivalent of the Imaginarium. As it has been imagined, it is contained by the Imaginarium but the Imaginarium can also be shown to be contained by it, therefore they are similar in that respect. Each contains the other and each contains itself infinitely many times, in infinitely many versions of itself. That being said the Box is probably more similar to the Omniumverse instead, making it slightly bigger than the Imaginarium. But it still exists inside the Imaginarium in the same way that the Omniumverse also exists inside it. The multiple Altarca existing in the Box easily correspond to different, very large, Imaginatas.
Although this view of equivalency with the Box assumes that the Box is a unsurpassable top level structure. If that is not the case, if one accepts Box transcendentalism than the equivalency with the Box no longer holds and the Imaginarium becomes larger than the Box. It becomes, instead, equivalent to whatever larger, absolute structure stands on the top level position instead. If no single structure is placed on a position of containing everything else by itself the Imaginarium will still contain all those structures, unless some, or all, exist on the Realium instead.
Imaginatas contain their own internal Cosmologies and they can even have multiple parallel Cosmologies at the same time, either related or unrelated to each other. They can have any size and contain anything, from a single Universe or smaller, a Multiverse, a Omniverse or even larger. It is even possible for an Imaginata to contain its own Box or its own, equally sized Imaginarium or even Omniumverse inside it. Despite that, they still exist inside the Imaginarium so they both contain and are contained by it. Even those largest ones end up succumbing to the pressure and corruption of the Imaginarium eventually, after enough eternities have passed inside them.
Relations with the Omniumverse, the Realium and Itself
The Imaginarium itself is part of the Imaginarium to the same extent that it is part of the Realium.
It is imagined therefore part of itself. It is both larger and smaller than itself at the same time. It contains itself infinitely many times. But imagination is, by all evidence, something real, therefore part of the Realium. To the extent that all we imagine is part of reality in a certain way, so are all of the Imaginatas and the Imaginarium itself. They are all real in the sense that they are really imagined. But the Imaginarium is less real than all of the Imaginatas inside it for its definition is that which is not real. This agrees with the impossibility of fully imagining it for that would make it real in this way. So the Imaginarium is the thing that cannot ever be fully real despite having to be real at the same time.
The contrary is also true. Many of the aspects of the Realium or even its entirety are also part of the Imaginarium. We imagine reality all the time, we perceive it and process those perceptions in our brains, our minds and those perceptions live inside the Imaginarium. Also, is there any difference between experiencing something and imagining it? Some insights point for the brain to be unable to tell the difference. Is the past real? We remember it but, as such it exists only as recordings and as memories in our minds, therefore it exists in the Imaginarium where all such things exist.
We make recordings of reality all the time. Are they recordings or are they the reality? They are real recordings but even if they are 100% truthful of what they record they don't record all of reality and they are not all of reality. They aren't even the part of reality that was recorded. The map is not the territory after all. Therefore they are not the same as reality, so they must be in the Imaginarium. Even more so if they are not 100% truthful in all the little details. And so they must for the writing is contained in reality so it cannot also contain reality at the same time. That contradiction is a property that can exist in the Imaginarium but not in the Realium, as far as we know. So the writing does not contain reality, does not describe it entirely and in 100% faithful way so it must be describing some Imaginata that is extremely closer to the Realium in the ways described in the writing. Any reading of a recording produces only an imaginary view of the reality, an Imaginata, not reality itself. There is a difference between information and what it describes. Information, even knowledge, at this level are just other words for imagination, they are part of the Imaginarium. Reality was recorded to create the recording but what is recorded is now only information which exists in the Imaginarium as another Imaginata. More than mere recordings of reality, works of fiction are clearly even more in the Imaginarium, much deeper and further away from the Realium.
Point of View Hypothesis
A curious view of it all is one consistent with that of the philosophy of modal realism. By that view all of the Imaginatas would be as real as the Realium. That also turns the Realium into just another Imaginata, making them indistinguishable. Realium becomes just another name for "my Imaginata" or Actual Reality and Imaginata becomes just another name for Reality. The Imaginarium becomes the entire Omniumverse and the space that exists between all the existing realities. There is probably no way of knowing if this is true or not though. But given that we are talking about the Imaginarium, truth is probably just another word for the locally imagined consensus.
Despite being very hard to define in many ways, there are a few properties that can be attributed to it.
Permeable to Imagination
It is very permeable to and molded by imagination. Things that are imagined appear here somewhere and any thoughts about it will cause it to change according to them. Strong or persistent thoughts will cause some area to stabilize in the form imagined and remain that way for some time.
It could be said that even this description of it is only valid because it is being imagined this way. This is a valid description of what the Shallow Imaginarium is but not of the entire Imaginarium for nothing can be said that applies globally to it all.
Any thoughts, ideas or imaginations exist here and stay here somewhere wandering around and never truly disappearing. They just become weaker and weaker and move farther and farther away.
The more stable areas inside it are called Imaginatas. No matter how badly defined an area is, how chaotic or subjective, how contradictory or impossible, any seed of imagination, belief, thought, reasoning or any other similar process will just give birth to a new Imaginata.
The area surrounding an Imaginata, or the Realium for that matter, is mostly defined by the agglomeration of all thoughts, ideas, beliefs and fantasies of the people from that place. But what shapes it strongly is what people believe it to be. If they imagine it as a chaotic sea of colors than that's probably what it will be or look like. It is also common for each area of it to function more or less like the world it surrounds for that is what people in that world are familiar with and what they will expect. Therefore whatever laws govern such a world, laws of physics or something else, will also tend to apply to some extent in the area of the Imaginarium surrounding that world. Despite all that, entering it can still be very confusing at the minimum or extremely dangerous at the worst case.
Any area is usually defined by what the people closer to it think or believe about it.
If contradictory beliefs exist, the closer area will be defined by those as well without any problems and will itself become contradictory in nature. It is also possible that the same area will be very different to different observers if each one imagines it to be something different. Sometimes it depends on the power behind the imagination. Sometimes an area ends up as the result of a mixture of all the multiple imaginations, sometimes it ends up being a different thing for each observer, sometimes something in between.
Even its subjectivity is subjective. Even its permeability to imagination or any of its properties are alterable by said imagination. It is subjectively subjective, not objectively subjective.
Because it is imagination, by analyzing it you only analyze yourself and your darkest and deepest corners. You might not like what you see in there!
It is possible to speculate that by analyzing the Imaginatas, their behavior and properties one analyzes the properties of the Imaginarium itself, even if in an indirect way. This idea postulates that the behavior and even existence of the Imaginatas are the direct result of some property or properties of the Imaginarium. This would seem logic at first sight but unfortunately logic does not apply to the Imaginarium at all.
It is probably ideas like these that cause a very large area of the Imaginarium, which is easily confused with its entirety as we tend to do ourselves, to behave in the way described here. This is the area called the Shallow Imaginarium.
Other areas probably behave very differently and present very different properties, maybe with Imaginatas behaving differently, having different types of Imaginatas, a different type of things, or maybe nothing at all. It is not actually possible to know for knowing is also imagining and imagining creates Imaginatas. It is a cycle and a contradiction at the same time that knowing the Imaginarium creates what is known about it. It is not possible to say what comes first, what is known or what is knowable for only that cycle exists. Both things come first and both come second.
The larger body of the Imaginarium is even less defined than these areas and there is a real risk that by talking about it we are already defining just another larger area instead of talking about the whole. It is important to remember that, by definition, the Imaginarium cannot be defined, that is its fundamental property. Actually, its fundamental property is that it has no fundamental property. Actually, its fundamental property is the contradiction in that sentence. Actually, its fundamental property just is. Actually, its fundamental property is not fundamental at all. Actually, its fundamental property is not even a property. And it refuses to allow some property, even a fundamental one, to define it in any way. Whenever a property or definition is found that applies to it, it is immediately shown to be just another larger Imaginata area inside it instead. Just so, just because.
All the worlds of imagination and fiction exist here. All our thoughts, dreams, ideas, beliefs, everything we reason, talk about or conceptualize.
A song that describes well some of what can be find in here:
Sweet dreams are made of this
Who am I to disagree?
I've traveled the world and the 7 seas
Everybody's lookin' for something
Some of them want to use you
Some of them want to get used by you
Some of them want to abuse you
Some of them want to be abused
These are all Imaginatas and they exist here like stars and galaxies exist on the Universe, like islands exist on the sea, like air bubbles in water, like dust particles in the wind. Large or small, they exist in all types and forms. Some are Universes, Multiverses or even larger, some are entire Cosmologies like those presented here, some are single galaxies, solar systems, even isolated planets or just smaller parts of them like small rocks floating in the void of chaos that surrounds them. They can even be as small as a single city, a building, a single room or even smaller.
They can have any form as well, not just those that would correspond to a place. They can be a vehicle, an item, a creature, even a sapient one, a single beating hearth without a body surrounding it, constantly pumping the currents of imagination around it.
They can be isolated dimensions without contents or things that exist without space for them to exist in. They can be partial universes where reality breaks apart randomly in several places, pocket realities of extremely limited size, even non physical things like concepts, words, sounds or smells, images, bursts of electricity or heat, a color that exists by itself without a canvas to be colored by it, a few lines of software code passing by or an emotion that has come to say hi. Even seeds of ideas never had by anyone can be found here somewhere.
Simple small ideas with weak existences float adrift on the sea of the imagination. Sometimes they collide with each other or group together forming more complex, larger, stronger ideas that become more powerful as they grow and draw others into them. Sometimes they seem to be attracted by some force akin to gravity but not quite the same. Sometimes the opposite happens and, all of a sudden, a complex idea just falls apart into its smaller, simpler, constituents. Other times a very strong, very solid idea just shoots through a larger more complex one tearing it apart into pieces. When a complex idea breaks apart into its simpler smaller components, it doesn't necessarily divide into the same individual components that joined together to form it in the first place (In the same way that one can get 4 by adding 2+2 but it is then possible to divide that same 4 into 3+1, the starting numbers and the final ones are different).
Many of the things inside can be very valuable and desired by many, to the point of actually being real treasures, capable of changing the lives of the lucky few able to find them.
Idea or concept mining is a common activity. This consists in collecting free floating ideas and concepts in order to make use of them somehow. Because it is the place of though and imagination, all thoughts and ideas occur there and can be found there. If one knows how to look it is possible to learn what anyone has ever though or believed about anything. Seeds of unborn ideas can be found floating around, some can be genial and collecting them could lead one to a breakthrough invention or a beyond masterwork art form. The solutions to all problems real or imaginary, including those yet to come. One with amnesia could find their lost memories in there as well. These things are even used as a form of currency on some places. Some ideas and concepts are obviously more valuable than others and those rarer ones can become very highly prized and be traded by very large amounts of currency or other resources.
Many other treasures and mysteries can also be found there. Small Imaginatas can be found, full of whatever riches or resources have been imagined for them. They open up the door to the possibility of endless resources of any type, from energy to raw materials, knowledge, technology, entertainment, all types of pleasures, new cultures and civilizations of any type and at any level or development.
Not just physical or plausible treasures either. It is easily possible to imagine that things like the fountain of youth or divinity seeds that would turn anyone into a god could be found here, therefore they can.
Completely abstract things can be found here directly. The love of a desired person that does not correspond such feelings. Happiness on its pure form. Things like a sense of humor or a larger intelligence. The power to do something one could not do before, like the power to fly or the power to become invisible, enhanced strength or the power to see again (useful for a blind person). All these things and more can be found here somewhere in many different forms, possibly, but not necessarily, assuming some quasi-physical substance.
Horrors of the imagination
In the same way that all treasures of the imagination can be found here, so can all its horrors, so one should thread very carefully, even when dealing with an apparently valuable finding for it can turn out to be something else entirely.
Even though any one can imagine whatever they want and it will be there somewhere, there is no guarantee that it will be in an easy to reach place. The more precious and valuable it is for the one imagining it, the harder to find and obtain it will probably be, possibly even being surrounded by dangers of different types, traps, guardian creatures, etc, all probably taken from the unconscious mind of the one imagining them in the first place. And it is only normal for the unconscious mind to link valuable or desired things to them being hard or dangerous to find and get or even to them being already owned by someone else who will want to keep them. Even the slightest seed of a doubt or a danger will contaminate the result therefore one requires the most extremely focused mind in order to create exactly what one desires without any flaws or drawbacks. The many unconscious thoughts of others related to and close enough to have an effect on the result will also affect it to some extent.
What can be found inside it usually possesses some or all of the properties of the Imaginarium itself.
Things are not always what they look like and they can also be very subjective in nature, changing what they are depending on the point of view, who the observer is or even one's beliefs about what is being observed. Many actually change what they are as soon as they are observed.
Something that appears to be beneficial may actually be very dangerous and something very precious may look extremely plain and boring. A creature may look like anything and what looks like a creature may actually be anything but. Sometimes things even look like exactly what they are.
Anyone capable of entering it will most likely need to take some way to ensure their survival with them for it has many dangers.
The only helping trait it has is that the expectations of those entering it will tend to form a stability bubble around them, which helps to some extent. If they are able to focus enough they can even shape the area around them to their desires. Other than that it is very much chaotic and subjective to an extreme.
Things inside of it
Despite many treasures and useful resources existing inside it, there are also many many things that are dangerous to the unexpected traveler.
The horrors and nightmares of entire civilizations roam around somewhere inside it. Any harmful, fearful, dark thought anyone ever had is in there somewhere and can be very dangerous in many different and unexpected ways.
Random thoughts can manifest themselves around the people who have them and even good things that one finds can change to become dangerous in some way if those around it are not careful with what they think about and how they think it.
Many things have become alive in some weird way. These are usually attracted to that property that really things have which makes them different from fantasy, being driven to feed on it. This will cause them to attack and prey on any travelers they can find to feed on them. Being a victim of such an attack causes one to have that property drained away. This is a similar effect to that described below on the section entitled Corrupting, except that it is directed, focused and much faster and obvious. The creatures doing it gain the absorbed property and are able to become partially or even fully real themselves in return, at least temporarily.
It has no space or time to exist in, no air to breathe, no ground to stand on, no gravity at all or anyway to move inside it. There are no points in space, no specific locations, one can keep on zooming in or out forever on the same place and move in that direction without moving.
Senses get mixed in and confused, sounds can become visible and colors turn into smells. What one person sees or experiences has no relation to what the others do, even when they are together. Because all the senses are mixed in, what one person sees, another one can hear and the next one smells and so on. Other times, all their experiences combine together to form a common but very confusing one. This can result on everybody in the group perceiving all the different and contradictory experiences at the same time or in a "reality by consensus" being defined for the group.
Distance is also subjective and has no meaning. A thing that is perceived as being at an extreme distance might be jut right next to you and something that seems close by can be at the most extreme of distances.
Even size is relative. A thing that is 1cm inside one Imaginata can be one parsec on another or even more. And when they meet in the Imaginarium they can appear to have any size relative to each other.
The only way to move around, if no space and solid ground is found, is by focusing one's thoughts into moving in the desired direction. But even direction is subjective, it is possible to look left and see oneself from the right or from any other point of view. It is even possible to see multiple images of oneself in several places at the same time, at different distances, all of them real.
Sometimes one can even see one's own past or future in several places. Going to those places just means coming back to the same point but at the different moments in time.
Sometimes space functions like time and time like space, sometimes one or the other does not exist at all. Sometimes they function in completely weird ways. Sometimes there are many more dimensions of one or the other or both.
As the Imaginarium has no time or can have multiply time dimensions in the same place, all passing at different speeds, it is not possible to know how much time as passed while inside it. A second can become a million years or the other way around.
The entire experience tends to cause extreme emotions of helplessness, confusion, many types of fears like claustrophobia, fear of open spaces, fear of heights, fear of people in general, different unnerving sensations like a feeling of being followed or observed, all of these eventually leading to some form of madness.
Even those entering it in their minds alone, not physically, are subjected to these effects. They might not be as dependent on a physical reality to move around and to deal with their environment as one who is there physically but their minds are still used to interact with the environment around them in a sort of physical way.
Its ultimate danger though is that it slowly consumes the existence of anything inside it, corroding it like an acid that leaves nothing unburned.
Real things slowly become less real, more subjective, less focused, less defined, eventually having their nature change into that of the Imaginarium itself, being turned into fantasy, subjectivity, imagination, becoming non-real.
Those affected by this tend not to notice as it is a slow process and there is nothing inside the Imaginarium that can serve as a point of comparison to remember oneself of what is real and what is not. Only when one tries to leave it again can this change become apparent as the contrast between the reality, even if only that of a stable Imaginata, and what one have become in whatever amount of time has passed. At that time it is already too late and nothing more can be done about it.
Resisting the dangers
The only way to resist these effects to some extent is by being able to focus one's thoughts enough but even this is a temporary measure for there is only so much time one can maintain a focused and continuous thought without being distracted by anything and if there is anything that the inside of the Imaginarium is is distracting.
Entering it requires leaving reality itself, going through the Great Barrier into the land of fantasy. This sentence alone should give an idea of how hard it is to achieve.
The easiest way to do it though, is through the mind. Any usage of the imagination in any way is already a form of entering it just slightly, like wetting one's toes on the beach.
The exact same thing that gives it shape is what allows us to reach into it. We do this all the time without even knowing it or thinking about it. We are able to explore large parts of it as well as many of the Imaginatas that exist in there. We are also able to remember our past which lives there through the use of our memory and to plan for the future by using our ability to predict it through expectations and reasoning that also allow us to cross the Great Barrier into it.
Sleeping and dreaming penetrates it much deeper, as does day dreaming.
Lucid dreaming is an even more powerful way of doing it. Its the form at which the potential of the Imaginarium to be molded by one's desires really becomes apparent. But it is also deceiving in that it hides the chaos that lies beneath it all and the difficulty in controlling it, for the unconscious mind keeps it stable for the entire process.
Meditation is similar but it allows one to reach as deep while remaining conscious.
Many hallucinogenic drugs also send one's mind into it, possibly providing some degree of stabilization around it, possibly not. The dangers inherent to the usage of such drugs are, many times, not actually directly derived from their usage but a consequence of being inside the Imaginarium.
Astral projection is probably the more advanced form of having one's mind leave the body and it also allows for easy entering into the Imaginarium, if one is aware of it and how to do it. It can become so easy as to the point of being trivial.
Entering it physically is much harder. One's body is part of one's reality and what needs to be made is to separate that part of reality that is one's body from the rest of it.
How does one get into the land of imagination? The story of a book, the setting of a TV show or a movie? How does one enter into the realm of all that is not real, the sea of the elements of imagination? This is probably impossible so, in order to do it one must achieve the impossible.
Magic is one way, the more obvious one, if it exists in one's reality at all. But some kind of super-science can also allow to achieve this result. It just needs to understand the nature of reality and be able to poke holes through it. How it can be done and how easy it is depends on the nature of the Imaginata one is trying to exit from.
It also crosses back into reality
More strange than being able to enter it is to have it or its contents, to have fantasy, cross back into reality and affect it.
Sometimes the Imaginarium itself or some Imaginata is able to cross the Great Barrier into the Realium at some level. This can be witnessed as we are affected by it happening, we have our feelings affected in this or that way because of it, we become convinced of something or get a new idea that just appeared out of no where, we get inspired, motivated, unmotivated, etc. All those things are examples of the Imaginarium reaching out to us, into the Realium.
Things physically entering reality from outside probably do not happen in the Realium itself but they can happen all the time in several Imaginatas. It all depends on the nature and permeability of the Great Barrier around them.
Reasons to enter it
There are many reasons that lead those crazy enough to do so. The first and most obvious one is because it is in our nature. It is there so it causes a need to explore it to appear. It is the last and larger frontier.
Some explorations are purely scientific in nature, driven by the search for knowledge, an attempt to understand the nature of this new frontier and what it can say about the nature of their own reality. Others are purely driven by greed in an attempt to collect as much of its endless resources and treasures as possible.
Attempts at contacting the realities of other Imaginatas are also typical. Some are simply motivated by curiosity, a desire for cultural exchange or exploration. Others by attempting to establish trading relations with them. But many are also motivated by a desire for conquest and exploitation of their resources.
Another common reason is a desire to escape one's own reality, to find something better or to create a new one shaped by one's desires. The idea that anyone can create and shape their own personal reality according to their whims and live like a king there or even become a god inside it can be very tempting.
It is even possible to change oneself by imagining something different, possibly something more but the risk is even higher.
Activities inside of it
Many civilizations from different Imaginatas are aware of the existence of the Imaginarium and some are able to enter it with more or less difficulty using different methods and processes.
There is some exploration of it, analogous to our own exploration of space.
Researchers study its apparent patterns and behaviors from the safety of an Imaginata, using special machines akin to telescopes that allow looking into it. They study other Imaginatas in the same way we study other stars or planets. They also study the brain and the process of imagination and relate those two disciplines for that is the nature of the Imaginarium. Some research stations have been establish inside it for scientific purposes.
The areas immediately surrounding an Imaginata have the properties of both that Imaginata and the Imaginarium. They are very stable and very similar to the Imaginata itself but not fully so. It becomes more unstable as one reaches farther away into the Imaginarium. These are the most explored areas, the places where most structures are built and where most activity exists. They are sometimes referred to as beachheads.
As different civilizations explore it and venture into it several end up contacting each other and forming different types of relationships.
Any contact with other Imaginatas usually tends to be mostly limited to the closest ones. They tend to have somewhat similar or, at least, familiar inner realities for they usually originate from each others imaginations and exist as derivations of each other. Because of this or as a reason for this they also tend to have people with similar ways of thinking or believing and imagining.
Politics of every kind ensue as the result of different civilizations meeting each other. They trade and learn with each other, form larger groups based on common interests, etc.
As it usually happens whenever different civilizations meet each other, there are also military incursions into it, prisons, military bases, fleets, war!
Many civilizations have built or attempted to build many types of lasting structures directly inside the Imaginarium, unattached to any Imaginata.
These structures serve many possible usages from exploratory and research stations, military outposts, mining facilities, etc. If some area of interest is found inside it, usually some type of structure will be built in order to try to claim and maintain control over it. Any found resource that one intends to collect also requires the necessary machines and structures to do so effectively. Industrial complexes that take advantage of some property of the Imaginarium in order to work more efficiently. Artificial habitats help dealing with over-population problems inside of Imaginatas that suffer from it. On places where the Imaginarium is sufficiently understood and explored, even tourist places can be found.
Due to the inherent orientation difficulties inside the Imaginarium another common type of structure is a navigational beacon, similar to a light house but adapted to the specific conditions of this much more hostile environment. They are intended to help ships reach their destinations much more consistently and faster. Without them any type of movement through the Imaginarium is much more uncertain and a large percentage of ships used to get lost. Even with their presence, disappearances are still not a rare occurrence.
Another structure intended to make movement more constant and safer is the establishment of routes of some type. These can vary from simple guiding lasers to the construction of actual physical road like structures. Due to their size and because, by their very nature, they are usually not entirely inside well stabilized and controlled areas. They are usually very hard to follow correctly unless one is very familiar with them and knows how to do so safely. It is not unheard of for them to change in unpredictable ways under the effects of the environment around them. Because of this they require regular maintenance and the ruins of ancient paths that fell into disuse can be found floating around sometimes. Even then, with all those flaws and limitations, they still make the paths along them much safer than without them. Despite these requirements they are very commonly used to establish trade routes between Imaginatas or as umbilical cords to the artificial structures constructed in no man's land.
All these artificial structures will require a form of maintaining their stability in face of the corrosive nature of the Imaginarium. The more they are, the larger they are, the closer they are to Imaginatas and the more people they have on them the more stable they become. Unless these structures exist in an area very close to several Imaginatas that provide it stability, these effects tend to not be enough for them to be entirely dependable. Because of this there have been many attempts at artificial stabilization technologies.
Lost and abandoned structures are not uncommon and are probably much more numerous than those in use and properly stabilized. Usually they are warped by their environment, having lost all or part of their stability. The most primitive ones didn't even have a stability generation mechanism, which is probably what caused their eventual collapse in the first place. Some can be in their current state due to past accidents that lead to their destruction. Many can be very dangerous and unpredictable.
Tunneling through it
Tunnels have been digged through the Imaginarium to link multiple Imaginatas together. The creation of portals between the realities of different Imaginatas usually manifests physically as tunnels crossing the Imaginarium space between them.
They allow for communication and movement between them without having to enter and be exposed to the Imaginarium directly. They are similar to roads except that they are closed to the Imaginarium environment outside of them, providing a much higher measure of protection.
Its their construction and maintenance that is the hard part but once they are in position things become a lot easier. They are much harder to build than simple roads for they require actually isolating the environment of the Imaginarium from entering them over their entire distance which is not easy. It is not simply possible to first build the open tunnel and then siphon out the Imaginarium from the inside of it. Not for such larger structures.
Some areas have very large and complex networks of those tunnels inside them linking hundreds, thousands, millions and, sometimes, even higher numbers of Imaginatas. The larger the network, the more tunnels there are and the more Imaginatas they connect to, the stabler is the area between those Imaginatas and the safer and less maintenance the tunnels require. This happens because so many closely related Imaginatas have a much stronger effect on the area around them and each one reinforces the others, all contributing together to stabilizing it. The closer and more related two Imaginatas become the more common, similar and related their imaginations tend to be as well. If a network becomes strong enough in the space between several Imaginatas, the entire grid becomes a huge common beachhead. In the extreme the close Imaginatas can even join together into a single larger one, covering the entire space between them all.
Other structures existing directly in the Imaginarium can also be linked to those tunnel networks.