Central Plexus
The central plexus was a device present on all Borg vessels.
The central plexus was used to connect a ship to all other ships within the Borg Collective. Because of its importance, the central plexus was protected with forcefields. - Class four tactical Borg vessels used multi-regenerative security grids to protect it, which made beaming someone into it impossible due to the heavy shielding.
Entering the vicinity of a central plexus was risky to say the least. The corridors leading to it were protected by forcefields. To enter the plexus the power grid of the section it was in could be brought offline, but this would bring an intrusion attempt to the attention of the Borg, or it could be accessed by the primary access port - which was isolated from the main powergrid. The access codes of this access port could be bypassed and after placing four actuators in their correct position the access port would open and so gave access to the central plexus room.
The central plexus itself was located in the middle of the room. After removing one of its side panels the plexus would be accessible. Borg assimilation tubules could be used to download information into the exposed plexus.
In 2377, Janeway, Torres, and Tuvok infiltrated the central plexus of Borg tactical cube 138 to release a nanovirus - to save the inhabitants of Unimatrix Zero. Because of the plexus interconnectity with other Borg vessels the virus was expected the travel fast into the Borg Collective and Hive mind. Within minutes this resulted in thousands of drones who were no longer under the control of the Borg Hive mind. (VOY: "Unimatrix Zero, Part I", "Unimatrix Zero, Part II")
When the insertion of the virus was discovered by the Borg Queen, she accessed the central plexus and the results of the nanovirus could be seen as flickering strands within the central plexus itself.