Lord Zack's Planescape
Most planar scholars agree, there is a lot they don’t know yet about the planes. For certain deep layers like Chronias and Nessus, little, if anything is known about them. In addition, some postulate the existence of unknown layers or planes, and some even claim to have gone there. The so-called Ordial Plane is a perpetual focus of rumors and attempts to find it.
Some have posited the existence of a so-called Far Realm where the laws of the universe all breakdown, and where various aberrations like the aboleths and illithids hail. Beyond this, some say, are entirely alien universes with their own set of planes. Some contend that the universe is self-contained, that so-called other universes are just misidentified parts of this one and that the Far Realm is nothing more than a distant part of the Ethereal Plane.