Dimension seeding

Dimension seeding was an invasion method used by the Cybermen of the Cyber-Mainframe to form bridgeheads into parallel universes and begin converting their inhabitants, (AUDIO: Code Silver) as part of their campaign to upgrade the multiverse. (AUDIO: Master of Worlds)

Virtual conversion
In order to form a connection from one universe to another, the Cybermen would translocate dimension bridge technology from a universe they already inhabited to one that they had yet to gain access to. (AUDIO: Code Silver) There were at least two different methods of performing a "virtual conversion" - cyber-conversion via dimension bridge technology - that were performed by the Machine, a security system that monitored the worlds assimilated into the Cyber-Mainframe and detected if mental avatars arrived on these worlds as a result of using dimension bridge technology.

Instantaneous conversion
Once a "candidate" used dimension bridge technology, which took the form of an advanced virtual reality headset, part of their consciousness was separated from their body and transmitted to a planet conquered by the Cybermen in a parallel universe. The Machine would attempt to attack the candidate via the use of different cyber-units, so as to use the candidate's mental avatar to form a dimension bridge to the parallel universe that their body was located in. Upon being attacked, the candidate's consciousness would be returned to their body shortly before undergoing partial cyber-conversion. (AUDIO: Telepresence) A dimension gate (AUDIO: Master of Worlds) would then form as the dimension bridge created from the candidate's consciousness connected the two universes.

Conversion over a dimensional gap
However, if a candidate had used dimension bridge technology for an extended period of time, but then disconnected from the technology, they could still be cyber-converted and used as part of a link between universes, although the conversion would take significantly longer than through direct use of the dimension bridge technology, (AUDIO: Telepresence) as the upgrades had to be broadcast across a dimensional gap. (AUDIO: Code Silver)

This method involved the Machine reestablishing its connection to the candidate and commencing a data transference, slowly broadcasting into the candidate's brain information from the Cyber-Mainframe necessary to establish a link between universes; due to the sheer amount of information broadcast from the Mainframe, this process was often quite painful for the candidate. The Machine was also capable of altering the candidate's nerve impulses so as to control their response to the pain inflicted by this process, and could detect if the candidate was aware of the process being performed on them and, by extension, render them unconscious.

If rendered unconscious, the candidate would wake up in a dream-like state, alongside an individual taken from the candidate's recent memories, who would assist the Machine in further establishing the link between universes, such as informing the Machine on the progress of the dimension bridge's establishment, and advising the Machine as to whether or not the data transference should be increased - as the data transfer risked informing the candidate of who was attacking them and their intent - or if the candidate's cerebral functions should be limited further. Eventually, the candidate would be attacked by Mechanising Worms, shortly before final conditioning on the candidate commenced. After waking from their dream-state, the candidate would be attacked again by Mechanising Worms, this time undergoing partial cyber-conversion and having their physical strength and connection to the Cyber-Mainframe boosted. This would lead to the completion of the candidate's full virtual conversion, allowing a dimension bridge to form between their universe and the universe that the Machine was making the connection from. (AUDIO: Telepresence)

Securing a foothold
After the newly-formed dimension bridge was connected between the two universes, a patrol of around ten Cybermen, led by a Cyber-Controller and a Cyber-Commander, would enter the new universe via the dimension bridge and attempt to secure a foothold in the new universe, starting with the general vicinity of the dimension gate, the endpoint of the dimension bridge. This process involved the Cybermen deleting the majority of the lifeforms in the immediate area whilst using mobile conversion units to gradually increase the Cybermen's numbers. Once the area was secured, the Cybermen would begin to spread across the planet, converting the rest of its inhabitants and securing further stronger footholds until they had eventually converted the entire planet. After this, full seeding would commence, which involved the Cybermen travelling to, and converting, further planets in the new universe, repeating their invasion methods until the entire universe was converted; due to how large a universe could be, full seeding could take thousands of years to complete. (AUDIO: Code Silver)

History
The Cybermen of the Cyber-Mainframe performed dimension seeding on an unknown number of universes, (AUDIO: Code Silver) as a key stage in their campaign to upgrade the entire multiverse. (AUDIO: Master of Worlds) One Cyber-Controller later remarked that the Cybermen had translocated dimension bridge technology to "many" universes, in preparation to commence dimension seeding elsewhere in the multiverse. (AUDIO: Code Silver) However, even in the universes that the Cybermen did eventually break into, they did not always perform dimension seeding successfully; as Petronella Osgood later noted, there were universes in which the Cybermen were overthrown by humanity before they could commence full seeding, others in which they failed to secure a foothold, and some universes where they were destroyed by solar radiation. (AUDIO: Master of Worlds)

N-Space was one of the universes the Cybermen attempted to perform dimension seeding in. (AUDIO: Code Silver) On Earth in the 2010s, the Auctioneers discovered dimension bridge technology that had been translocated by the Cybermen from another universe. Unaware of the technology's true purpose, its creators or how it was powered, the Auctioneers developed a smart phone virtual reality game out of the dimension bridge technology to make money out of, and forcefully used Lieutenant Sam Bishop of UNIT as a template to develop the game. After Kate Stewart and Osgood rescued Sam, (AUDIO: Game Theory) they took the dimension bridge technology to Fortress Island, where Osgood discovered the true function of the technology.

Together, Osgood, Colonel Shindi and Josh Carter used the dimension bridge technology to travel mentally to a parallel Earth conquered by the Cybermen. After the trio encountered several malnourished mutant dogs, Kate attempted to disconnect the dimension bridge technology, but was briefly rendered unconscious by a huge electrical shock from the technology, although she soon recovered. Upon detecting their mental avatars on the surface, the Machine attempted to upgrade Osgood, Shindi and Josh, using a large Mechanising Worm, a cyber-converted mutated dog, and a large flock of converted pigeons, but was initially unsuccessful. After Osgood, Shindi and Josh entered an underground train station at Piccadilly Circus, the trio eventually found a large cavern filled with thousands of human batteries. The Machine then confronted the group and unleashed several Mechanising Worms on the trio, causing Osgood and Shindi to undergo virtual conversion; however, Josh was unaffected (AUDIO: Telepresence) due to his Auton physiology scrambling the control signal, which had been calibrated for human physiology. (AUDIO: Code Silver)

Meanwhile, Sam Bishop, who was recovering in Fortress Island's medical bay (AUDIO: Telepresence) after being forcibly exposed to the dimension bridge technology for an extended period of time by the Auctioneers, (AUDIO: Game Theory) experienced a delayed virtual conversion caused by the Machine, who reestablished its connection to Sam, before commencing a data transfer and rendering Sam unconscious. In a dream state, Sam encountered a pseudo-Lieutenant Parsons, who assisted the Machine in establishing a link to N-Space through Sam's mind. After being attacked by a swarm of Mechanising Worms, Sam awoke in the medical bay, before another swarm of Mechanising Worms partially upgraded him. Although Sam was able to escape the medical bay and reach Kate, shortly after arriving, the Machine completed his virtual conversion. On the parallel Earth, the Machine then activated Hive Section 49, (AUDIO: Telepresence) a patrol of ten Cybermen, including a Cyber-Controller and a Cyber-Commander. (AUDIO: Code Silver) The converted Osgood and Shindi then used the equipment in the laboratory to link the newly-formed dimension bridge from the parallel Earth to N-Space, (AUDIO: Telepresence) allowing the activated Cybermen to cross the dimension bridge and confront Kate and Josh, forcing the pair to flee.

While Kate and Josh broadcast a Code Silver alert across Fortress Island, the Cyber-Controller ordered the Cybermen to take full control of the fort, activating numerous mobile conversion units to assist, and allowed Osgood to reprogram the Auctioneers' virtual reality game so as to speed up the conversion of the Earth of N-Space. As the Cybermen engaged the fort's soldiers in battle, Kate and Josh locked down the entire fort and activated a communications shield, preventing the Cybermen from broadcasting beyond their own shield. Despite their attempts to regain control of the fort, Kate and Josh were unable to stop Osgood from broadcasting the reprogrammed game across the planet, allowing the Cybermen to virtually convert millions of people globally within seconds. With Josh seemingly upgraded by a mobile conversion unit, Kate attempted to detonate a nuclear warhead underneath Fortress Island. (AUDIO: Code Silver)