Mondas

Mondas was the original homeworld of the Cybermen of this universe and also the twin planet of Earth, which, in ancient times, moved from its old position out into space.

Geography
Mondas was identical to Earth even down to the size and shapes of its continents, albeit the continents are up-side-down and the surface water patters of the earth are replicated in inverse, vice-versa for the land (if there was South America on Earth on Mondas it would be up-side-down and it would be an ocean in the shape of South America.) (DW: The Tenth Planet)

''Given what we know about the geology of Earth and the formation of its landmasses this is exceptionally unlikely and strongly suggests an unknown, vastly powerful agency somehow influenced the development of either or both planets.

Native life
Originally, the Mondasians resembled the humans of Earth in every respect. (DW: The Tenth Planet) One account shows that in the same way that parallels to humans existed on Mondas, Mondasian Silurians and Sea Devils shared the planet with dinosaurs and Cybermen. (DWM: The Cybermen: "The Dead Heart") A reptilian god figure, Golgoth, also lived on Mondas at this time. (DWM: The Cybermen: "The Dark Flame")

Early history
Originally, Earth and Mondas occupied the same orbit in the Sol System. It was stated that "Mondas" is another name for Earth in one of the ancient languages (DW: The Tenth Planet).

One account says Mondos was called Marinus and this planet was vastly evolved due to Worldshaper devices to a point where the planet had completely changed and became Mondas. (DWM: The World Shapers)


 * This implies either contact between the two twin planets or else a degree of parallel evolution, with similar languages evolving on both worlds.

During a time when parts of Mondas had a tropical climate, Cybermen met and warred with Silurians and Sea Devils. (DWM: The Cybermen: "The Dead Heart") The godlike reptilian humanoid Golgoth appeared and destroyed all but one Cyberman and finally destroyed the lone survivor. (DWM: The Cybermen)

Some centuries after, human-appearing Mondasians of roughly 20th century technological development had appeared and Mondas had begun to move out of its orbit. Archaeologists had discovered remnants of the Cybermen. (DWM: The Cybermen: "The Ugly Underneath")


 * This last story implied that the discovery of the extinct Cybermen would lead to the Mondasians deciding to use Cybertechnology, beginning the cycle of creation anew and creating a new race of Cybermen.

Separation from Earth orbit
Mondas left that orbit and drifted on a journey to "the edge of space" (The Tenth Planet). This was likely the result of a cosmological event, see the Notes section for a full list of theories on precisely what that event may have been. Although the edge of space implies a journey to the edge of the universe it is generally accepted that Mondas's journey was somewhat shorter; as there was no indication that Mondas was capable of the necessary speeds to complete such a journey in mere millions of years.
 * Regardless of distance travelled it seems unlikely that Mondas travelled to another star system, or indeed to a point where it was closer to another star than it was to Sol as the Cybermen, being driven by will to survive and operating logically, would have chosen to travel to the closest suitable energy source to replenish Mondas.

The Struggle for Survival
As Mondas travelled away from the life giving warmth of Sol conditions on the planet grew steadily worse. This forced the inhabitants to relocate underground in order to survive (BFA: Spare Parts). Before Mondas left the solar system a faction of Mondassians began to explore the possibilities of cyber conversion, possibly inspired by the remains of Cybermen discovered earlier in Mondassian history. This faction is believed to have left Mondas to settle on Planet Fourteen, the then 14th planet in the Sol star system and gradually converted themselves into Cybermen (Doctor Who: Cybermen).


 * One account of the Cybermen's creation shows the Voord of the planet Marinus using stolen time technology to "quick-evolve" themselves into the earliest Cybermen (DWM: The World Shapers). This could be the story of the creation of Cybermen on planet 14 or of another race of cybermen not tracing it's origins back to Mondas

As Mondas travelled beyond the outermost planets conditions on the planet continued to deteriorate. The Doctor and Nyssa arrived on Mondas prior to the rise of the Cybermen. There, they discovered the Mondasians living underground in a culture virtually identical to that of England in the 1950s, although with more advanced technology. They witnessed those Mondasians who had remained on their planet the slowly slide towards cyber-conversion, as the cyber-faction had. The Mondasian cyber-conversion program was conceived by the cybernetic gestalt intelligence controlling Mondas as a solution to the planet's increasingly desperate situation. (BFA: Spare Parts)


 * The gestalt intelligence had similarities to the Cyber-Planner.

The Return and Destruction of Mondas
As Mondas continued to drift away from Sol it became apparent that the planet's energy would inevitably be depleted and that the only recourse was to use Mondas's last remaining energy to return to the Earth's orbit and plunder her energy; to that end the Cybermen had developed a means of propulsion for Mondas itself. This allowed them to pilot the planet through space, arriving back in its original orbit By 1986. The Cybermen invaded the South Pole while Mondas drew closer, draining Earth's energy to replace and supplement its own. However, the energy absorbed was too much, and Mondas disintegrated. The destruction of Mondas destroyed all of the Cybermen on Mondas and also killed all of the Cybermen on Earth, who were dependent upon Mondas for energy. (DW: The Tenth Planet)

The "New Mondas"
In 1988, the Cyber-Fleet travelled to Earth, planning to take the Nemesis statue, a Time Lord weapon that would allow them to conquer Earth. The Cyber-Leader planned for there to be a new and final era where imagination, thought, freedom, and pleasure would end. Humanity would be enslaved and Earth would become the Cybermen's base planet: the "New Mondas".

The Doctor, however, launched the Nemesis in space to meet with the Cyber-Fleet. When it did, it exploded and destroyed the entire fleet. Earth remained unchanged.(DW: Silver Nemesis)