A few notes on prime world accessibility.

Properties of deep primes.

Here's a bit more.

Well first on this subject one must say that the properties of so-called deep prime worlds are just as varied, and perhaps are more varied as those that are shallow. The only similarity that connects them all is the fact that it is harder to planeshift or gate to them than other primes.

One thing that has been noticed in the exploration of deep primes is the shunting off of them when a planeshift is not successful. The shift goes off but instead brings the caster to a similar shallow or shallower primeworld. Indeed there have been several groups of deep primes found that are very similar to one another, right down to the geography and even individuals living on those worlds. These primes appear to be alternate versions of each other, and are among some of the hardest for our scholars to reach.
There is the very real chance that our explorers have visited more primes that have alternates and simply have not discovered the alternate versions of them yet. Another, hotly debated theory suggests that perhaps every plane has alternate versions of itself. There may even be alternate versions of Sigil and the upper planes, though there are rumors about this so far no credible evidence has come to light. One noticeable tendency among the more easily reachable alternate primes is a propensity towards chronomancy and time travel. Perhaps the practice causes fractures in the fabric of the plane eventually leading to the prime splitting into two different worlds.

Another startling discovery with alternate primes is the fact that they exist even for shallow planes. One notable one is for the prime of Torril. The shallow version is known in some circles for producing a few very powerful mages, though the plane is relatively indistinctive in other respects. However the alternate version that has been found seems to be connected to an entirely separate cosmology. There seems to be bleed over in the two primes as the older and less refined texts on planar travel in the shallow Torril seem to refer to the deeper's cosmology.