From my readings of the various magic supplements the general attitude seems to be that you can view the worlds mana level as a grid laying "on top" of our reality. Local mana levels are fluctuations in this grid, sort of like what you see in a sound visualization system. But whatever.
Before the ages of magic these fluctuations were very low and even localized "spikes" never went very far. If you were billy badass you could attempt to locally "raise" your mana level but for the most part nothing you did could raise the mana level appreciably.
Now, you could view things like the astral planes as "planes" lying above our reality. Stacked as you were on top of us. These "spaces" exist in higher dimensions that we are not capable of perceiving any more then we can "see" hypercubes as anything but shadows in our 3-dimensional world.
As the overall mana level rises (like an ocean influenced by the moon) you will start seeing more and more "spikes" poke through into the astral plane 'above' us. Once you spike into it you can get a "connection feed" to our normal universe and draw mana. The so-called spike babies are simply local spikes that due to the manaflow they had expressed their metagenes. Taken in this context they would have had to been careful where they went and what they did, since they were local abberations they could have possibly entered a low mana level that even their heightened 'local' mana level would not have enabled them to maintain a connection to the astral, and thus they would "shift" back to a normal human (Changeling anyone?).
The Awakening was the point where the local background level was high enough that it poked through the astral plane everywhere. As the Awakening approached the number of "spikes" would have been getting higher and higher and some areas where the mana level was generally higher would have been completely 'Awakened' before others. There would have been no set 'date' that the Awakening actually happened, though the data set by the Mayan Long Count could have been a good guess for when the general mana level was high enough to touch the astral through most of the world.
Since the real world now "touched" the astral you could project yourself into. Imagine it like being in a very very tall room with multiple floors placed high above you. The floor below you represents the mana level. The floor would be uneven in places, some high and some lower then others. This represents local variations in the mana level. You can jump ("spike") around on the floor but if the mana level is too low you can't reach the floors above you no matter what (first floor is the astral plane in this example). As the mana level rises it's like moving the floor up. There comes a point where you can jump up and pull yourself onto a floor even though the rest of the floor has not reached that level yet (spike babies). Hanging on is hard though and if the floor drops you may lose your grip and fall back down.
In Shadowrun the floor has reached the astral level and is still moving up. In a separate, adjacent building you have the Horrors and other nasties sitting around. They are pretty "high" in their separate buildings and to reach our building someone needs to build a path over to them. Since it's hard to build a bridge up as well as across it's usually only when the mana level 'floor' reaches the point where the bridge only needs to be so long that the Horrors just jump over, the bridge does not have to go all the way since the Horrors can jump pretty far, especially the weaker ones who can jump across at lower mana levels. Recent SR events would be like blowing up the bridge then sealing the exits with concrete. Now even when the Horrors jump over they have to chip away at the wall. And by then the level of mana would ahve fallen so low none of them could survive.
Example, Invae and low level Horrors live in the adjacent building
- but on the lower floors. The guys on the higher floors can't
go down that low since there is not enough energy to sustain them.
Now you have the floor in the other building going up, and the
low level baddies notice this. Then you have the Great Ghost Dance
which throws a bridge out pretty far across and upwards. The baddies
in the next building jump over onto the friendly bridge into our
building. They can survive at our mana levels and start doing
their thing. The bad boys on the top floor notice this bridge
that lets them get across a lot sooner then would normally be
possible (since the bridge goes 'up' as well). They start using
low level grunts who can jump across at low levels to start getting
others to extend the bridge and make it wider (some of those Horrors
are 'fat' you know). Aztechnology is the contractor for
this job (contractors will do anything for money!). Since the
mana level is still rising and the bridge gets higher with it
as well as through the manipulations of the Horrors buddies the
setting is set for the baddies to jump over (though at a weakened
level) and start going about their dirty business way before the
floor got to their level normally. Why can't they jump from above
to below? Silly, Horrors are afraid of heights and very fat, so
if they jumped they would fall down Between the buildings and
go *splat*.
Hmm that's a good idea for talking about Horrors. They don't really NEED mana to survive. But they are so heavy that crossing the gap between the buildings they need a bridge between them. The more powerful the Horror the fatter he is and the less distance he can jump. Lighter Horrors like wraiths and the Invae( not horrors but they live in the next apartment) can jump a lot farther.
That's a gross simplification, since it would be more like you raise yourself into astral "space" like rising into the atmosphere. The metaplanes are higher up in the astral plane, but the local mana level keeps getting higher, making jumps into that area easier and easier (mages jump really high).
Another way of looking at it, and this could tie in ED, is that the astral plane is slightly 'tough'. Even though we've reached the level that everyone is connected to the astral layer ("touching" it) it's like a rubbery layer that we are poking into. Mages are "pointier" then normal people and penetrate into it, whereas the mundanes are "blunt" and while the astral touches them they can't penetrate into it. As the mana level rises even the blunt people will start poking through or making rips (hence in ED everyone can use some magic). Make sense?
Mana voids are areas where the mana floor has fallen below the astral "level" and thus there is no connection anymore. Awakened races spike high enough to not have be overly affected by most voids but I bet if they stick around or encounter a strong void they could regress to their low-mana counterpart (human). It's not a black hole or anything, just a lack of connection to the astral. Space has no mana level at all so it's always a void. This does not mean you can't cast magic but you have to be a drekhot magician to "jump" high enough to touch the astral again (and jumping can be tiring!!). In this context the lack of life really has nothing to do with it except that lifeforms add to the mana background count since they filter astral energies through them (the higher the mana level the more energy life "radiates").