Mondas

Mondas was the twin planet of Earth and original homeworld of the Cybermen of the Doctor's universe. In ancient times, it 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 were upside down, relative to Earth. However, it had no visible clouds in its atmosphere. (DW: The Tenth Planet)

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) Golgoth, a bipedal reptilian god in physical form, possibly linked to the Silurians, also lived on Mondas at this time. (DWM: The Cybermen: The Dark Flame)

Creation
Mondas was created as an experiment by the Constructors of Destiny as an experiment of sorts to see how it would differ from Earth. (PDA: The Quantum Archangel) Mondas had, like Earth, at one time, an intelligent reptile species.

Origins of the Cybermen
One account says that Mondas originated as Marinus, a planet previously visited by the Doctor, the Cybermen having "quick-evolved" from the Voord. (DWM: The World Shapers)


 * See Creation of the Cybermen

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


 * This implies a high degree of parallel evolution and/or contact between the two 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) 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 account 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.

Evolution of the Cybermen
Mondas left that orbit and, for unknown reasons, drifted on a journey to "the edge of space", (DW: 'The Tenth Planet), 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. (BFA: Spare Parts). One Cybermen faction, the CyberMondasians decided to stay on their homeworld. Another, group, the CyberFaction more committed to total cybernetic conversaion, pushed outwards in the solar system and colonized Planet 14. (NA: Iceberg)

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

Return of Mondas
Their only recourse now was to use Mondas's last remaining energy to return to the Earth's orbit and plunder her energy. In 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. The energy drain also had an adverse effect on The Doctor's health causing him to regenerate into his second incarnation (DW: 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.

Origin of the Cybermen
The Cybermen origin story in The World Shapers portrays Marinus, Mondas, and the mysterious Planet 14 referenced in The Invasion as the same place. Lance Parkin in the un-official work AHistory has rationalised this away by speculating that Cybermen may have originated independently on different planets at different times. At any rate, other stories set in the Doctor Who Universe don't take The World Shapers" into account. 

Iceberg and Doctor Who: Cybermen, by David Banks, who had also played the Cyber-Leader on Doctor Who, explain it differently.


 * See discussion in Creation of the Cybermen article

Origin of Mondas itself
State of Change, which has an artificially created duplicate of Earth called Terra Nova, may possibly hint that Mondas once went by that name, but The Quantum Archangel explains the matter away unambiguously by saying that the Constructors of Destiny had created two duplicate planets as an experiment.

Other matters
The date of Mondas' departure from its original orbit is not known, nor is the full extent of its wanderings through space. It is widely believed (and Spare Parts concurs) that Mondas' journey "to the edge of space" involved it physically leaving the Solar system and travelling through the galaxy. However as it apparently did not possess a propulsion system at the time, it is unknown how the planet could have attained sufficient velocity to cover interstellar distances in a period apparently of only a few thousand years.