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A transwarp hub in the Delta Quadrant

Four of the six hubs, one in each quadrant

A transwarp hub was a megastructure that connected the Borg Collective's galactic network of transwarp conduits. Connecting to thousands of exit apertures in all four quadrants, the hub allowed the Borg to quickly deploy vessels almost anywhere in the galaxy in minutes; giving them a decisive tactical advantage. The galaxy had six such hubs, characterized by extremely high neutrino emissions and intermittent graviton flux. The hubs were supported by interspatial manifolds, whose shielding the Borg Queen regulated from the central nexus.


The transwarp hub encountered by Voyager explodes

In 2378, the crew of USS Voyager discovered a hub in the Delta Quadrant, inside a nebula in grid 986. While Voyager was outside of the nebula, Ensign Kim and Seven of Nine detected high neutrino emissions accompanied by an intermittent graviton flux on long range sensors, leading them to speculate that the hub was a concentration of wormholes. The nebula contained at least forty-seven Borg cubes. Collaborating with Captain Janeway's counterpart from twenty-six years in the future, who had traveled back in time to change history, the Voyager crew destroyed the hub and, with it, the entire transwarp network. Admiral Janeway had deliberately infected herself with a neurolytic pathogen and allowed the Borg Queen to assimilate her, disrupting the Queen's ability to regulate the shielding around the interspatial manifolds. Having taken the ship into one of the hub's conduits, the Voyager crew then fired transphasic torpedoes, causing a chain reaction that destroyed the entire network. Staying ahead of the resultant shock wave, Voyager exited the aperture less than a light year away from Earth before the conduit collapsed. (VOY: "Endgame")

Although six hubs are said to exist, only four appear on the map in Voyager's astrometrics lab.
For "Endgame", the design of a transwarp hub was worked on by Foundation Imaging. "Jarrod Davis worked out that big, huge, evil-looking transwarp hub," reported Foundation's Visual Effects Supervisor, Robert Bonchune. "This is another case where I was really nervous about the effect beforehand [....] But it turned out really great. Paramount asked for only a minor change; they felt the shot we had done initially was a little too white, so we subdued that a little bit and made it darker." (Star Trek Monthly issue 82, p. 18)
According to the Star Trek Online video game, the Federation established the first links of its own system of transwarp conduits in 2399, and by 2409, would link the Sol system to key starbases in and around Federation territory, including Deep Space 9, Deep Space Station K-7, and Starbase 39-Sierra. The Federation network does not appear to employ a visible hub, though a Federation-built transwarp aperture complex located near Risa leads to Borg-controlled territory. The Klingon Empire also maintains or at least uses a transwarp network, though it is unknown when it was established relative to the Federation version.