Marinus

Marinus, also known as Planet 14 and possibly Mondas, (COMIC: The World Shapers) was a water-world in the Voora Marinii group. (PROSE: Interference - Book Two)

History
At the time the First Doctor visited with Susan, Barbara Wright and Ian Chesterton, the Conscience of Marinus machine was hidden on Marinus, and there were acid seas and glass beaches on the planet. By then, it has also been invaded by the Voord, under the leadership of Yartek. (TV: The Keys of Marinus, AUDIO: Domain of the Voord)

The Daleks came to Marinus and fought the Voords until they learned of the Voords' knowledge of the Great Power and joined forces with them. They captured the First Doctor, who had also travelled to Marinus. (PROSE: Doctor Who and the Daleks)

Designated as Planet 14 and given a worldshaper at some point, the Voord took control of the worldshaper and rapid-evolved themselves. When activated, thousands upon thousands of years passed and the oceans dried within a week, with the Voord mutated into Cybermen. When the Sixth Doctor intervened, an aged Jamie McCrimmon sacrificed his life to destroy the worldshaper, which caused time to accelerate and seasons to change in seconds with massive geological activity. By this time Marinus was barren, yet the worldshaper's last energy pulse completed the evolution of the Voord into Cybermen. (COMIC: The World Shapers) During year A10%? of the Last Great Time War, Marinus was attacked by the Daleks. The War Doctor built a Chronic tripwire to defend the planet. Because of the paradoxes and shifts in causation during the Time War, Marinus lost all its ecosystems and was left with only its acid deserts, but conversely, the Voord evolved into stronger beings. Though the Time Lords possessed the power to revert the damage to their planet by changing the course of history, the Voord themselves feared they would also be reverted to the way they used to be. Unsure he would be able to take care of this problem, the Doctor posed that he might be able to act as intermediary between the Voord and Time Lords once the war was over to prevent this from happening, given that the Time Lords would still be willing to listen. (COMIC: Four Doctors)

Behind the scenes
The World Shapers complicates the continuity of Doctor Who, though Cyberman history is complicated enough without this. Marinus, according to this account, becomes Mondas, and would therefore make Marinus the twin planet of Earth. It also means that Mondas is the second planet to be seen in Doctor Who proper, after Skaro.