A virtual world where some Borg drones lived as individuals during regeneration, the birthplace of a resistance movement aided by Voyager in 2377.
Unimatrix Zero is a shared virtual realm reached in the minds of certain Borg drones. Drones with a rare genetic mutation could enter it during their regeneration cycles, recovering their individuality and appearing as their former selves. There they lived as free people in a peaceful forest landscape, but on waking they forgot everything and returned to the Collective.
Seven of Nine had visited Unimatrix Zero during her time as a drone, and had loved a man named Axum there. In 2377 Axum contacted her, and the Borg Queen was hunting the realm, killing drones to destroy it. Captain Janeway, Tuvok and B'Elanna Torres were assimilated on purpose to spread a nanovirus that let the drones remember their time in Unimatrix Zero once they woke. The realm itself was lost in the struggle, but the drones who could now remember it kept their individuality and formed a resistance inside the Collective.
Unimatrix Zero shows a hidden longing for freedom within the Borg. It is one of the series' boldest attacks on the Collective and set up the resistance that troubled the Queen in later episodes.