Indeed, Jor-El of Krypton-One recognized the probability of a parallel planet to his own, one where he may be able to save himself, his family and his entire civilization. Devising a projector that would transport individuals across the dimensional void to the vibratory coordinates wherein he calculated that Krypton-Two would reside, he instead found himself trapped in the Phantom Zone. The same Phantom Zone that would become the prison for his world's most corrupt convicts. An interesting aside, the fact that Jor-El was generally correct in his analytical estimates, including his unfinished thought that perhaps Krypton-Two no longer existed (it hadn't, destroyed decades previous) seems to indicate that the extra-dimensional entity known as Aethyr that manifested itself as the Phantom Zone existed between these two vibratory planes. Indeed, this account indicates that it bridged the interdimensional void to the locale where Krypton-Two once was, sensing catastrophe like that which occured when two galaxies merged spawning Aethyr itself. Aethyr's doppleganger, the dimensional plane known as the Fourth Dimension, existed in Earth-Two's universe although it was discovered not by Zor-L but by some Earth-Two megalomaniacs like Zor, Anton Unreal, Bandar and Kalthoon. And the Phantom Zone also tied into other universes, as decades ago Jur-Ll of Krypton-Three journeyed from there into the Kandor of Krypton-One!
Aethyr eventually merged with Mister Mxyzptlk to torment Kal-El one final time, unleashing the Phantom Zone criminals before absorbing them once more into its pocket-dimensional realm. Meanwhile, Mxyzptlk's counterpart Mister Mxyztplk remained in good standing in the fifth dimensional land of Zrfff as its surrogate ruler, unlike his twin who was a perpetual perpetrator of pranks. The Earth-Two Mxyztplk kept what little sanity he had while the aeons old entity of Aethyr was no more when Earth-One's Mxyzptlk absorbed him. At this point, the Phantom Zone as it was constituted ceased to exist (although due to its timeless nature, the modern day version was accessed in the 30th century when its one good resident, Mon-El, was freed by the Legion of Super-Heroes). Meanwhile, Zrfff attached to Earth-One was destroyed while its Earth-Two twin remained a vibrant world in its own 5th dimension.
