Mondas

Mondas was the twin planet of Earth and original homeworld of the Cybermen of the Doctor's universe. In ancient times, it was dislodged from its old position out into space when the Moon was formed. (TV: The Tenth Planet)

Geography
Mondas was identical to Earth, even down to the size and shapes of its continents. When it approached the Earth in 1986, the planet was oriented such that its continents were upside down relative to Earth. However, it had no visible clouds in its atmosphere at that time. (TV: The Tenth Planet)

Native life
Originally, the Mondasians resembled the humans of Earth in every respect. (TV: The Tenth Planet) One account shows that in the same way that parallels to humans existed on Mondas, Lizard Kings and Sea Devils shared the planet with dinosaurs and Cybermen. (COMIC: The Cybermen: The Dead Heart) 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)

Creation
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) Mondas had, like Earth, at one time, an intelligent reptile species. (COMIC: The Cybermen)

Origins of the Cybermen


One account said that 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) Another account claimed that a group of Cybermen called the Faction later left Mondas to colonise Planet 14. (PROSE: Iceberg)

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)

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

Evolution of the Cybermen
Mondas left that orbit and drifted on a journey to "the edge of space", (TV: The Tenth Planet) caused by the sudden arrival of a moon between Earth and Mondas. This upset the gravitational equilibrium of Mondas, causing it to escape its orbit.

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 December 1986, the Cybermen invaded 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)

Origins of the concept
The idea of a parallel Earth opposite in orbit to Earth itself has occurred in a lot 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.

Cybermen
According to the non-narrative reference guide, Doctor Who: Cybermen, which this wiki does not consider to be a valid source, the drive which 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.