What is the Mar?
It's easier to answer the question of what the Mar isn't than what it is. Called 'the Weft' or 'the Synapse' in Lariat, 'anterograde' in Arkosy, and 'the Suture' in the twin cities of Pri-Mid, even the words used to describe it seem varied and indecisive, almost as if each one is referring to a seperate phenomena. Early Pass linguist Linus Vierordt was once cited as saying "The Mar may be a universal concept, but it is, without a doubt, the most difficult word to translate when reckoning with a new language.". Based on my own reckonings with unknown languages, I'd have to agree. This still doesn't answer the question, however.
I like to think of the Mar as the space between the muscle and the bone.
Here's an analogy I quite like. For much of the history of humanity, back to the earliest records we can find, there is a common conception of an 'underworld'. There is the reality of underground spaces - the caves, tunnels, mines, basements, boreholes, tombs, bunkers, foundations, etc that one can physically inhabit, and there is the conceptual presence in the mind of what these places are representative of, the realms of death, spirituality, connection, liminality, archive, and protection. The Mar makes the conceptual realm the physical. To enter the Mar is to enter a basement expecting to see only a basement, and instead to see it's history, perception, and possibility; to see a basement as it percieves itself.
The Mar is omnipresent in the world, a supra-physical aspect of any existing space. It is more concentrated in certain places and 'thinner' in others, likely based on the strength of these places histories and the weight they occupied in the mind in ages long past, though this is only one theory. See excerpts from Niven, Cooper, Pareto below:
"...were it true, in fact, that the relative density of fibrils was somehow controlled by a sense of the history of a place, for what reason is Oiler's Pass not completely unlivable, or, for that matter, Arkosy? It seems, even, that the inverse is true: fibrils are more sparse in highly populated areas, especially those that have been highly populated for longer time periods. The more poetic amongst us might be tempted to say it is becuase an ephemeral 'we' know these areas well, that to witness them is to call them to truth, but I am not as inclined to believe this. It is more likely, in fact, that it is the inverse: that no two people see the city the same, that the city, divided amongst the many eyes of it's citizens, becomes more real as the mar has less room to play it's own games of chance with the city's image..."
- Niven, Mar-theory and it's practice, [REDACTED] IO
"We call it the thread of fate, the weave of fate, the elusive red string of fate, all for a reason: The mar is in all likelyhood a binding agent of the world as we know it, a grand connecter between past and present, known and unknown, real and unreal - the broken and mended, the rent and repaired, the inconsequential and the grand, the understood and the mysterious, the author and the [WORD UNKNOWN]. How then, is it as dangerous as it is? To draw together unlike things, to reduce the space between an object and it's inverse to nothing is an act in which one of the two is obliterated, or, in reality, in which both are obliterated and the resultant object is the 'synthesis' that philosophers and their ilk discuss...the mar is, then, a space that becomes what is brought into it..."
- Cooper, The Poetics of the Twin to the World, published this year.
"An unknown? An unknown is a variable in an equation, not a space which devours all that is in front of it, a place that is, by all understandings of the world, alive. The mar is as much unknown as the Morganic Sea is: which is to say, it is both known and dangerous, understood and unpredictable. It can be measured and mapped, the pervasive myth of the mar as a place of mystery is a holdover from the early days of the world. The mar is, instead, a living thing. A neighbor on this continent. A particularly hungry beast. To treat it as anything else is to allow the beast to creep closer to the fields..."
- Pareto, All Falsehoods, 501 IO.
(Notice again the large variation in language. Though I like the similarities between Niven's 'fibrils' and Cooper's 'threads'.)
It is generally understood that the Mar is the force by which the world has ended so many times. It is also the force by which the past of the world is preserved. Although fear of the Mar is common, it is my belief - and the belief of a small handful of others - that exploration of the Mar is vital, and far less dangerous than the common perception holds.
It is almost certain that the next apocalypse will come no matter what we do. Why not see what we can find down there first?