Interstitial vortex
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An interstitial vortex linked the universe of the Doctor and another Earth which the sorceress Morgaine came from. The Seventh Doctor and Ace jumped into it to reach her for their final showdown. Apparently, they were allowed to cross it thanks to the intercession of the Destroyer. (TV: Battlefield) The Seventh Doctor was afraid that passing unshielded through this kind of vortex would have been dangerous, because the forces loose inside could rip someone into molecules. (PROSE: Battlefield)
