Mondas

Mondas was the twin planet of Earth and original homeworld of the Cybermen. In ancient times, it was dislodged from its old position out into space when the Moon was formed. The Cybermen eventually learned to pilot their planet and took Mondas back to Earth, where it was destroyed.

Native life
Originally, the Mondasians resembled the humans of Earth in every respect. (TV: The Tenth Planet, AUDIO: Spare Parts) Some accounts show that in the same way that parallels to humans existed on Mondas, Lizard Kings and Sea Devils shared the planet with dinosaurs, Cybermen, (COMIC: The Dead Heart) and apes. (COMIC: The Prodigal Returns) Golgoth, a bipedal reptilian god in physical form, also lived on Mondas at this time. (COMIC: The Cybermen)

The Museum of the Last Ones had specimens of extinct species from Mondas. (PROSE: The Last Dodo)

Geography
Mondas was identical to Earth, even down to the size and shapes of its continents. The one major way that it differed to Earth was that its continents were upside down in relation to Earth. However, it had no visible clouds in its atmosphere at that time. (TV: The Tenth Planet)

Creation
According to one account, Mondas originated as Marinus, a planet previously visited by the First Doctor, the Cybermen having "quick-evolved" from the Voord. This account claimed that Marinus was known as Planet 14. (COMIC: The World Shapers) This was contradicted by the Twelfth Doctor, who believed that while Cybermen did evolve on Marinus, they originated separately from the Mondasian Cybermen, through parallel evolution. (TV: The Doctor Falls)

According to another account, Mondas was created by the Constructors of Destiny as an experiment of sorts to see how it would differ from Earth. (PROSE: The Quantum Archangel)

Originally, Earth and Mondas occupied the same orbit in the solar system. It was stated that "Mondas" was another name for Earth in one of the ancient languages. (TV: The Tenth Planet)

Under Lizard King rule
Mondas had, like Earth, at one time, an intelligent reptile species. (COMIC: The Cybermen)

A cosmic cloud once appeared near Mondas. A Cyberman from the future came through the cloud and was captured by the Lizard Kings. At this point, the Lizard Kings had never encountered Cybermen before. They were intruiged by the cloud the Cyberman came from and sent a modified ape-servant through the cloud to investigate if it was a viable way to escape the rogue planet which was approaching Mondas fast.

The Lizard Kings never left Mondas, (COMIC: The Prodigal Returns) and the sudden arrival of a moon between Earth and Mondas caused an upset in the gravitational equilibrium of Mondas, causing it to escape its orbit (AUDIO: Spare Parts) and begin drifting on a journey to "the edge of space". (TV: The Tenth Planet)

While parts of Mondas still had a tropical climate, the Lizard Kings encountered the Cybermen of their own time and warred with them. (COMIC: The Dead Heart) Golgoth appeared and destroyed all but one Cyberman and finally destroyed the lone survivor. (COMIC: The Cybermen: ?)

By the time that Mondas was well and truly separated from Earth, the Lizard Kings had fallen and the Cybermen became the new rulers of Mondas. (COMIC: The Prodigal Returns)

Evolution of the Cybermen
A cosmic cloud appeared near Mondas, providing a temporal link to its past. After an ape modified by the Lizard Kings came through the cloud and was captured by Cybermen, they became curious about the cloud and decided to send a scout through it see if there were more apes that could be converted into Cybermen on the other side. (COMIC: The Prodigal Returns)

Some centuries after, Cybermen had been wiped out and human-appearing Mondasians of roughly 20th century technological development had replaced them. Archaeologists discovered remnants of the Cybermen. (COMIC: The Ugly Underneath)

Further 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. They had by this time developed a society parallel to that of 1950s England. The Cybermen took over and enforced cyber-conversion for the surviving Mondasians. (AUDIO: Spare Parts) One Cybermen faction, the Mondans, decided to stay on their homeworld. Another group, the Faction, more committed to total cybernetic conversion, pushed outwards in the solar system and colonised Planet 14. (PROSE: Iceberg)

The Mondans developed a drive which used electro-magnetic energy to push the whole planet through space. (PROSE: Iceberg)

Return of Mondas
Their only recourse now was to use Mondas' last remaining energy to return to the Earth's orbit and plunder her energy. (TV: The Tenth Planet)

In 1873, Mondas was located 200 light-years from Earth. (AUDIO: The Silver Turk)

In 1903, after receiving a wealth of information from the future, Grigori Rasputin foresaw the return of Mondas. (AUDIO: The Wanderer)

In 1984, the Sixth Doctor left a damaged Cyber-Leader from the far future on Mondas knowing that it would be destroyed with the rest of the planet two years later. (AUDIO: The Reaping)

In December 1986, the Cybermen invaded the Snowcap tracking base in Antarctica 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. As the planet plundered Earth's energy, the First Doctor's life-force was caught up in the maelstrom, triggering his regeneration into his second incarnation. (TV: The Tenth Planet) The Sixth Doctor would note Mondas' destruction as being part of the Web of Time as he thwarted an attempt made by future Cybermen in the previous year to save Mondas. (TV: Attack of the Cybermen)

The official statement on Earth attempted to cover up the Cyberman invasion by claiming Mondas was an asteroid, known as "the Mondas Asteroid", which went supernova before colliding with Earth. During Earth's war with the Myloki between 2066 and 2068, Alex Storm found UN documents which revealed that Mondas was actually a planet. However, these documents were destroyed by "some ignorant clerk" to "save space". (PROSE: The Indestructible Man)

Origins of the concept
The idea of a parallel Earth opposite in orbit to Earth itself has occurred in pre-modern astronomy (Counter-Earth) and some works of science fiction, though modern astronomers consider this unfeasible due to the orbital mechanics involved. The idea had earlier almost appeared in Doctor Who in the proposed Malcolm Hulke story The Hidden Planet. Apart from this, the idea occurs in countless comics, movies, novels and short stories.

Continuity conflicts and ambiguities
The Cybermen origin story in The World Shapers portrays Marinus, Mondas, and the mysterious Planet 14 referenced in The Invasion as the same place.

Iceberg and Doctor Who: Cybermen by David Banks, who had also played the Cyber-Leader on Doctor Who, explain it differently. According to the later, non-narrative reference guide, which this wiki does not consider to be a valid source, the drive that propelled Mondas through space was developed during the journey to "the edge of space" in order to effect a controlled return to their original orbit.