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When Karazhan was first announced, there were rumors that an upside-down version of the tower existed, extending down from the bottom floor, deep underground. Whether or not Blizzard actually plans to implement this 'Lower Karazhan' remains unknown.
It is likely that this is a reference to The Last Guardian by Jeff Grubb, a novel which details the last weeks of (living) activity within the tower. Several references are made to an "inverse" of Karazhan which begins at the lowest level and works its way downward, with Medivh's secret chambers lying at the very bottom.
There exists a building behind the nearby graveyard in which there is a stair downward towards a locked gate. In past, a player could go through this gate as a ghost to explore the large empty area beyond.
Archmage Alvareux wrote a treatise, proposing the existence of an exact copy of Karazhan in a world other than Azeroth and that both somehow share the same space, which might be a reference to lower Karazhan.