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 had 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 learnt 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, and 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.

After the Time War ended with the disappearance of both the Time Lords and the Daleks, the Voord remained on Marinus in peace until they heard the call of the Time Lords from a pocket universe. Fearing that the Time Lords would return and strip away their newfound power, the Voord salvaged technology from the downed saucer and used it to build an engine that hid them in their own pocket universe, disappearing more effectively than the Time Lords ever did. As a result of this, the universe forgot about Marinus, even the Doctor having only vague memories of his adventure there, most notably the acidic sea, but being unable to identify why he knew such things.

The Voord were eventually found by an alternate Twelfth Doctor. Driven mad by his solitude, the Doctor joined the Voord and became their leader, masterminding a trap so he would ensure his own future. Manipulating Clara Oswald, the alternate Doctor led his past self, as well as his tenth and eleventh incarnations, to Marinus where they fell victim to a continuity bomb, eventually emerging in the Voord city. On account of a miniaturized Weeping Angel, Gabby Gonzalez was able to warn her past self of these events. When the new version of the travellers emerged in the Voord city, they convinced the alternate Twelfth Doctor to stand down which forced him to revert the Voord and return them to Marinus, likewise reduced to its former state. The Eleventh Doctor, however, decided to speak to some alien friends of his and push for Marinus to receive aid for the larger galactic community. (COMIC: Four Doctors)

Behind the scenes
The World Shapers complicates the continuity of Doctor Who. According to this account, Marinus becomes Mondas, making Marinus the twin planet of Earth. It also means that Mondas is the second planet to be seen in Doctor Who proper, after Skaro. This discrepancy is referenced in TV: The Doctor Falls, which states that the Cybermen evolved on the planets Mondas, Planet 14 and Marinus, separately.