1) Where is it mentioned? Is it mentioned in any of the 2nd ed Planescape products, or is it a theory postulated by Planescape fans alone?
2) What would the Ordial be like?
Actually, now I have another question. Three questions.
3) Why is it that the Ordial hasn't been discovered?
My own feeling is that to understand the Ordial, we must first look at the nature of the other transitive planes and the planes they connect.
I belive that everything started in the Inner Planes. The Positive Energy Plane, a well of infinite creative force, spontaneously birthed the elemental planes. These, in turn, formed the building blocks of existence and life. Through the Ethereal, these elements slowly spread to the Prime, where, under natural physical laws, they formed planets and crystal spheres. Whether life first developed on the Prime or the Inner Planes, I do not know.
With life in existence, sentience also emerged, and with it belief. This belief moved through the medium of the Astral and the Outer Planes, previously a blank cnavas, began to take form, at first very simply and then, as the believers beliefs became more complex, so did the Outer Planes.
But where on earth did all this stuff on the Outer Planes come from? Is it truly made of belief alone? I think it is unlikely. Ash from Gehenna isn't just pure belief. It is ash, the same substance you'd find on the Quasielemental Plane of Ash. Likewise, the Styx and Oceanus are water, and whilst both are influenced profoundly by the beliefs assoicated with the Lower and Upper planes respectively, both are actual physical water. In my opinion, the Outer Planes are not made of belief as such. It is just that belief, rather than usual physical laws, govern the the actual physical substance of the Outer Planes.
But the matter that makes up the Outer Planes has to somehow get there from the Inner Planes. I doubt it comes all the way via the Ethereal, Prime and Astral. Besides, since the Astral is a realm of pure thought, it is utterly unsuitable as the natural medium for the transportation of matter to the Outer Planes. Only the Astral conduits seem naturally suited to transporting matter from the Prime to the Outer Planes, and they are far from being as ubiquitous as seems necessary to transport the entirity of the infinite Outer Planes' physical essence.
The link, therefore, must be the Ordial. It transports matter directly from the Inner Planes to the Outer Planes, where it is shaped according to mortal belief. The Ordial is, like the Inner Planes, unrefined matter. It is unshaped until it arrives at its destination. This might make it somewhat similar to the Ethereal I suppose, in so far as the Astral and Ethereal and already similar (you know, kinda silvery white...) but it likely has the highly morphic trait and maybe, if and where it borders the Outer Plane, it takes on the alignment traits of the plane it borders as well as starting to coalesce into the appropriate matter.
As to why the Ordial has remained undiscovered for so long, I don't know.
1) Where is it mentioned? Is it mentioned in any of the 2nd ed Planescape products, or is it a theory postulated by Planescape fans alone?
It's a fan-only theory, based on the idea that if you see two transitive planes - the Ethereal and Astral - there must be a third (Rule of Threes). And also the Unity of Rings premise says that it's likely that the planes are a ring, so there must be a way to, once you've gone from the Inner Planes, to the Material Plane, to the Outer Planes, a way to continue on and get to the Inner Planes again without having to backtrack.
The idea comes originally from Mimir's Mapping the Infinite - http://mimir.net/mapinfinity/index.html
The entry for the chososion in the third Planescape Monstrous Compendium mentions an unknown plane that intersects the Inner Planes. It calls this plane the Macrocosm, and suggests it might be a bridge to an unknown multiverse. It could certainly be the Ordial Plane, too. Monte Cook said that although he wasn't aware of the Ordial Plane theory, he was indeed thinking of something similar to that. An unknown plane, anyway.
2) What would the Ordial be like?
The Ethereal Plane is the source of dreams and planes. New planes for from the primal ylem of the Waveless Sea; presumedly, the inner planes came there as well. But the Ethereal Plane is only potential - the Inner Planes are actuality. The Astral is where thoughts and dreams and gods go to die.
Therefore, I've decided that the Ordial Plane is a place of transformation, where matter becomes thought, where reality becomes concept, where forgotten ideas become living spirits, where the thoughts in your head become the raw stuff of the multiverse. Chris Nichols thinks it might be the place where Platonic forms reside.
What this would look like, I'm not sure. On the Planewalker splash page it looked like a series of multicolored tubes, channeling elemental energy to the Outer Planes and alignment energy to the Inner Planes. Whether it would need to be infinite or not is unclear. It might be very small - maybe just a three foot by three foot cube.
There's another page devoted to the Ordial here: http://pages.prodigy.net/shamilto/ordial/oplane.htm
It suggests the plane is the home of pure spirit.
3) Why is it that the Ordial hasn't been discovered?
Because there aren't any portals leading to it. Most spells don't let you reach planes that you've never been to. Hell, no one even knows the Ordial Plane's proper name.
It's possible it's drifted away from the rest of the cosmology, for reasons of its own.
Corsica Ralopolis, in the Mapping the Infinite site, attempted to reach the Ordial using a planar tuning fork. She returned, moments later, with no memory of her journey. Her body was carved with unexplained cuts, spelling out symbols in an unknown language.
I belive that everything started in the Inner Planes. The Positive Energy Plane, a well of infinite creative force, spontaneously birthed the elemental planes.
This is possible, although the fact that the Positive Energy Plane isn't still churning out new planes today makes it unlikely. In fact, positive energy is more likely to destroy solid matter than to create it, causing it to burst with too much life and become pure energy. The only products of Positive Energy that tend to last are crystalline in nature.
The substance of the Ethereal Plane, on the other hand, is continuing to form into new demi-planes, which are believed to eventually evolve into full planes (the Plane of Shadow is offered as an example of this, though in theory all planes were once demiplanes, at least all planes connected to the Ethereal). The Believers of the Source certainly associate the Ethereal with their Source.
But where on earth did all this stuff on the Outer Planes come from? Is it truly made of belief alone?
Yes. Well, belief and the substance of dead and ascendant petitioners, and that's essentially the same thing - belief, faith, souls, and gods are all essentially different phases of the same substance. There is no true space or time in the Astral Plane, and thus not in the Outer Planes (which, as you note, are probably formed from astral energies). Without space or time, there is no room for true matter. What the Outer Planes have is the concept of space, time, and form, which only exist in the Astral Plane after they've died. A traveler from the Material Plane who goes to the Astral is transmuted into pure thought; if the traveler goes to the Outer Planes she becomes pure concept. The same thing happens to any nonliving products moved from the planes of matter to the planes of belief.
Ash from Gehenna isn't just pure belief.
It's pure evil, tainted slightly with law, in the form of ash. If you take it to the Material Plane, it will corrupt or kill any plants you try to grow in it. Only lower planar foliage will thrive in such stuff.
If ash is brought to Gehenna from the inner or material planes, that ash is not pure evil; it's the idea of ash in solid form. If it's brought from another outer plane, it's likely the stuff of that plane's alignment in ash shape. Though it may be corrupted with prolonged exposure to Gehenna, it will always be another substance.
But the matter that makes up the Outer Planes has to somehow get there from the Inner Planes.
It could also be from the Material Plane. And remember that portals can link two distant planes directly, without passing through anything else in between. For example, Moradin's realm has portals to the planes of Earth and Fire (On Hallowed Ground).
the Astral and Ethereal and already similar (you know, kinda silvery white...)
The Ethereal Plane is a universe of multicolored mists.
"...the Ethereal's like a great fog-bound realm with mists of green, red, silver, blue and whatnot in between." (A DM Guide to the Planes, page 23)
"Streams of vapor emerge from nothingness. Swirling masses of mist - first blue, then aquamarine, then emerald swirled with crimson, all in continuous and ever-changing patterns - shroud a body's vision. Worls of fog and shadowy shapes of solidity float in and out of view. A few blobs of solid ether lazily sail end over end, some gaining density and others losing it." {A Guide to the Ethereal Plane, page 6}
And the boundary between the Border Ethereal and the Deep is a shimmering wall of color.
Ethereal protomatter is generally white, with bluish highlights.
The Astral is a deep storm-gray hue that glistens and shines like dew on a spider-web. Er... actually, everything just says it's silver in color, although the dew/spiderweb part is from A Guide to the Astral Plane. It might be like being inside a big steel bowl, with stars winking in the distance.
But it's in the nature of us poor sentients to try and break everything we don't immediately comprehend into neat little models - rings, triples, what have you - and pretend we "get it". Then of course, we fight each other over different interpretation of the poorly grasped darks. The whole "we-get-it-better-then-you'll-ever-get-it" thing is so obviously pointless that it actually makes the Guvners' laughable quest for knowledge look good.
(For a while now I've been meaning to hand in my Sinker avatar and sign up with the Bleak Cabal, but I said to myself "What's the point? It's not as if anyone would notice...or should care...")