Everywhere I've read of berks who have climbed on to rooftops and equiped fly spells to look passed the walls of Sigil, and every single one of them comes back with the same response, "There's nothing there."
Then the truly barmy who jump into this nothingness are said to be transported to a random plane. Yet has it ever been said anywhere of anyone returning? I don't think so! (I could be wrong about that)
I present an explanation of what this nothingness is. It is Byss, the Dimension of Entropy!
Yes, it is a dimension, NOT a plane my friends. What's the difference you ask? A dimension would be defined as more encompassing than a plane. The Outer Planes all occupy the same dimension, but this is a difference dimension from the one that the Material Plane occupies, and far different from the one that the Inner Planes occupy.
The difference between Byss, the dimension of Entropy and the three other known dimensions is that it is not segmented into different planes. Even the Material Plane has its cracks and holes. The Byss can be said to be a plane sized sphere of annihilation, perfect in shape and with but one single blemish that I will describe momentarily. It is not the source, but the sink of entropy, where everything, matter, belief, petitioners, planars and everything in between all go when they are broken down beyond their simplest parts.
Hold your fire my Doomguard brothers, I've got a good explanation for where your precious Negative Energy Plane fits into all this. You see, the Negative Energy Plane is just that. Negative Energy. It is the act of approaching entropy but not true entropy in a state of stability. Why else would it constantly be draining life to any who go there instead of just immediately disintegrating them?
You may be asking, if this Byss is a plane-sized sphere of annihilation, why aren't we all dead? Why hasn't the universe been vaporized already? The answer is right in front of your face my friends. As I've mentioned earlier, the dimension of Byss is the nothingness that surrounds Sigil. The city occupies a very special location in which it acts as a stopper on the entropic bottle so to speak. It is the barrier between Byss and the rest of the universe, and prevents the dimension of entropy from immedietely destroying all of existance.
But there is more to the story! The barrier that Sigil forms is not perfect! Entropy seeps in through any location it can find. These droplets of entropy become creatures such as the unelemental and the void dragons, true creatures of entropy.
My time is beginning to run short, but I wish to leave you with one last thought. The planes are filled with a a near infinite amount of...something, and the idea of a whole lot of Nothing might not seem too intimidating, but from what I have learned, a whole lot of Nothing is infinitely scarier than all the Something this Universe has to offer.
-Milo Sagthys, Doomguard heretic giving a talk at the Civic Festhall
I can not take credit for the name Byss or the idea of dimensions. They were presented in Craig Cochrane's book Immortals Handbook: Epic Bestiary. The Unelemental and Void Dragons mentioned as also stated in his boo. I simply got the idea of placing Byss around Sigil, as the emptyness never seemed to have too much of a story associated with it and the idea of a plane-sized sphere of annihilation seemed to work well.