Unimatrix Zero is only accessible to drones with a certain genetic mutation that allowed them to visit while they regenerate. Approximately one drone in every million carried that gene, and so had access. Unimatrix Zero was destroyed in 2377.
Five of Twelve, Secondary Adjunct of Trimatrix 942, was stationed on a scoutship along the boarder of fluidic space in a remote sector of the Beta Quadrant. His original identity of Axum was only retained during his time in Unimatrix Zero. While he was in Unimatrix Zero, he had a relationship with Seven of Nine. It was Axum that was able to contact Seven three years after her separation from the Collective. He hoped that she would be able to assist in saving Unimatrix Zero.
Along with the other drones of Unimatrix Zero, Axum had developed a nanovirus that would prevent the Borg Queen detecting them. However, the nanovirus had to be released in the real world to have effect. Janeway persuaded Axum to change the plan to one that would lead to the destruction of Unimatrix Zero, while allowing the drones involved to retaining their memories and individuality while remaining drones. Axum, along with Seven of Nine, was in Unimatrix Zero when it was finally destroyed.
Four of Twelve, Subjunction of Unimatrix 525
This Borg drone had been part of Unimatrix Zero until the Borg Queen discovered his gene mutation that connected him to it. She attempted to obtain from him the interlink frequency that bound all the Unimatrix Zero drones together, but he claimed not to remember. The Queen then dismantled him to analyse his cortical node. After the dismantling of this drone, the Queen said, "It's a shame you're not alive to experience disembodiment. It's the epitome of perfection."
Korok was an assimilated Klingon, who had the gene that allowed him access to Unimatrix Zero. He was sceptical about Axum bringing outsiders into their attempt to protect Unimatrix Zero from the Borg Queen. He was persuaded to join the plan to destroy Unimatrix Zero. Because of the efforts of the Voyager crew, he was able to retain his memory and individuality after Unimatrix Zero was destroyed and was able to take over the Borg Sphere on which he was stationed, and assisted Voyager in defeating a Borg Tactical Cube. He later joined other liberated drones with the hope of forming a resistance within the Collective.
Laura was a Human who was assimilated at the Federation-Borg battle at Wolf 359. It was only after the Voyager crew ensured the retention of memory and individuality of all the Unimatrix Zero drones that she assisted in it’s destruction. It is assumed that afterwards, Laura assisted in forming a resistance movement within the Collective.