Operation Fort Knox was the name that Captain Kathryn Janeway gave to the mission to retrieve a transwarp coil from a Borg sphere in 2375. While planning the theft of the transwarp coil, Captain Janeway used Fort Knox as a metaphor to explain the scale and the level of security around the prize they sought.
After studying a Borg data node containing tactical information regarding the movements of Borg vessels, Seven of Nine presented the information to Captain Janeway, Commander Chakotay, and Lieutenant Commander Tuvok in astrometrics. When she pointed out three Borg cubes along their current route, Janeway noticed another vessel, not far from their present location. Seven determined the vessel to be a badly-damaged Borg sphere, heading back to Borg space at low warp. It was then that Janeway got the idea to assault the sphere and steal a transwarp coil.
The mission briefing took place shortly thereafter, during which Janeway decided on the name for the mission: "Operation Fort Knox", after the impenetrable 20th century Earth facility. A plan was decided on in which they would head for the sphere and use a diversion to allow an away team to beam aboard undetected. The away team would then steal the coil and return it to Voyager. Chakotay recommended they run holodeck simulations of the mission, in order to cover every possible contingency as Lieutenant B'Elanna Torres offered some advice on masking the ship's warp trail. The plan was agreed on, and the briefing concluded.
In preparation, the crew of the USS Voyager ran several simulations, testing various strategies against the Borg. The plan involved Tuvok and Ensign Harry Kim making their way to the sphere's shield generators and detonating spatial charges, while Janeway and Seven of Nine proceeded after the transwarp coil. However, the initial simulations failed, as the away team fell twelve seconds behind schedule, allowing the Borg the two minutes they needed to regenerate their sensor grid and beam over to Voyager.
However, while reading some of her parents' field notes about the Borg, Seven of Nine discovered a method of hiding within a Borg vessel for prolonged periods of time. The bio-dampener, created by the Hansens, would cloak the away team's life signs by emulating the environment of the sphere, thus allowing them to complete their mission.
The first part of the plan was put into effect; a remote-piloted shuttlecraft was flown towards the Borg vessel emitting false sensor readings indicating there were three lifeforms on board. The intention was to allow the Borg to capture it, and, while the sphere's shields were down, give the away team an opportunity to beam in. This, however, did not go according to plan as the Borg did not consider the shuttlecraft a threat and simply ignored it. The shuttle needed to appear more of a target to the Borg. Therefore, Janeway ordered Chakotay to have the energy output of the shuttle's warp signature increased. The Borg took the bait and the sphere's shields were lowered, allowing the away team the opportunity to beam in.
From there, the mission went according to plan, with one unexpected turn of event. Tuvok and Kim made their way to the shield generators and planted their spatial charges in the appropriate places, while Janeway and Seven arrived at the transwarp coil and planted transporter pattern enhancers around the device, in preparation for transport. The spatial charges were successfully detonated and the coil beamed aboard Voyager. The away team then headed back to the transport co-ordinates for beam-out. However, en route to the transport site, Seven of Nine was contacted by the Borg Queen and told that Voyager would be destroyed unless she rejoined the Collective. Before Janeway could do anything, a force field was erected and she was forced to leave Seven behind.
Although the transwarp coil was successfully taken from the Borg, Seven of Nine had remained behind. Refusing to accept the idea that Seven left willingly, Janeway ordered the Delta Flyer be equipped with the transwarp coil and a rescue mission was launched in an effort to bring Seven back to Voyager. The mission was ultimately successful when Seven was retrieved from the Borg unicomplex. Afterwards, the transwarp coil propelled Voyager an additional 20,000 light years closer to the Alpha Quadrant before giving out, cutting roughly fifteen years off her journey. (VOY: "Dark Frontier")