The narrow passage joining the universe to its antimatter counterpart, serving as a safety valve between them, in which the two Lazaruses are finally sealed together for eternity in The Alternative Factor.
The negative magnetic corridor is the link between two parallel universes, one of matter and one of antimatter, in The Original Series episode The Alternative Factor in 2267. Within it the two kinds of matter can meet without destroying one another, so that it acts as a buffer keeping both universes safe.
The Enterprise detects a moment in which everything in the galaxy seems to blink out of existence, and traces it to a barren planet and a man named Lazarus. He is in fact two men who exchange places through the corridor: a raving Lazarus from one, bent on killing his opposite, and a calm, sane Lazarus from the other. Should the two ever meet outside the corridor, both universes would be annihilated.
Kirk forces the mad Lazarus into the passage, where his counterpart is waiting, and the Enterprise destroys the craft that serves as the doorway, leaving the pair locked in combat.