Unimatrix Zero
Unimatrix Zero was a virtual construct, and resistance movement, created by a group of Borg drones.
After it was shut down, drones formerly connected to Unimatrix Zero continued to resist the Borg Collective.
How Unimatrix Zero came into existence was unclear. Some believed it started with a random malfunction, during the assimilation of a single drone, which eventually spread to other drones.
This construct was created by, or maintained by, Borg with a recessive genetic mutation. The genetic mutation was rare, and only one in a million drones possessed it. This gene gave them the capability to create a collective, virtual world. This world was separate and hidden from the hive mind, and allowed the drones to act as individuals. It was accessible while drone regenerated in their alcoves, a sort of "dream world" for Borg.
Although this mutation made it possible for a drone to participate in Unimatrix Zero, they only were aware of it as long as they were in it. Entering Unimatrix Zero was only possible when a drone entered its regenerative cycle. When a drone's regenerative cycle ended, it could not remember the experiences gained while inside, but did remember them once re-entering. For non-Borg, entering was only possible via a mind meld with a drone in its regenerative cycle. The number of Borg drones who made up Unimatrix Zero were estimated to be about twelve thousand.
One Borg / former Borg with this mutation was Seven of Nine, who was able to enter Unimatrix Zero even after she was freed from the Collective.
While in Unimatrix Zero, a drone could choose to assume the appearance they had before being assimilated. A drone was aware when its regenerative cycle would end, giving it the opportunity to say goodbye to others before exiting. Although a drone could be killed within Unimatrix Zero, he or she would enter it again during their next regenerative cycle.
Unimatrix Zero was undetected by the Borg Collective until 2376. The Borg Queen found their interlink frequency, and could sense drone with the genetic mutation, which she saw as an illness. However, this method was slow, and would take centuries to find every drone with the mutation. Using the interlink frequency, the Queen was able to send drones to Unimatrix Zero to attack it.
Seven of Nine was called back into Unimatrix Zero for help - they had devised a nanovirus that would mask their genetic mutation, therefore hiding them from the Queen. Being the only Borg with the Unimatrix Zero mutation, and no longer part of the Collective, Seven was the only one who could retain the memory of Unimatrix Zero when she wasn't regenerating.
Although the Borg Queen warned Captain Janeway against interfering, Janeway and the USS Voyager crew continued to help Seven in her endeavor. After the nanovirus was released, the Borg Queen began to lose contact with drones possessing the genetic mutation. In an attempt to force Captain Janeway to give her an antidote for the nanovirus, the Queen interrogated her and destroyed entire Borg vessels in which she could no longer hear only two or three drones. Despite this, Janeway still refused.
To save the drones with the genetic mutation, it was necessary to destroy Unimatrix Zero. Though the Borg-free construct was gone, its destruction also resulted in the creation of a 'fleet' of multiple Borg vessels, under the control of former drones. These drones made up the resistance movement against the Borg.
It is unknown if any of those drones succeeded.
