Central plexus
From Star Trek : Freedom's Wiki
The Central Plexus is a device found on ALL Borg vessels, it's purpose being to link each ship to every other ship operating within the collective itself. Due to the nature of it's purpose, and it's importance as a means of maintaining the link by which the Borg operate, it is protected by multi-regenerative security grids and is always found deep within the centre of each ship.
