The Caves of Androzani
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Androzani Minor was a dry, sandy planet and the twin planet to Androzani Major.
Androzani Minor was a desert planet, its oceans having dried up billions of years ago. Due to the unusual composition of the planet, boiling hot mud flowed throughout the planet’s crust, blasting its way to the surface in dangerous explosions. The planet was riddled with caves (actually blowholes) that had been excavated by the violent mud flows, some of which lacked oxygen. Despite the conditions of the planet, it had a breathable atmosphere and was home to such life as the queen bats and the Magma Beasts, but was otherwise unsuitable for habitation by humans. (The Caves of Androzani)
When humans first colonised the solar system Androzani Major and Minor were located, they didn’t bother with Androzani Minor and focused on Major instead because of the desert climate and the near constant mud burst. It wasn’t until the humans discovered that Androzani Minor contained a drug in spectrox which granted near eternal life that they took an interest in it. (The Caves of Androzani)
In the 30th century, Androzani Major was part of the Earth Empire. (So Vile a Sin)
Androzani Minor was colonised by humans to gather the spectrox found in the caves. They used androids to harvest it. The supply was controlled by Morgus and Sharaz Jek, but Morgus betrayed Sharaz and caused him to be scarred by a mud burst. Sharaz tried to fight back with his androids. They were countered by Morgus’ federal forces. The Fifth Doctor and Peri Brown happened to land on Androzani Minor at this time. They were caught up in the conflict and developed spectrox toxaemia. By the end of the conflict, Sharaz, Morgus and his men were killed. The Doctor managed to find a queen bat and extract her milk, the only known cure for spectrox toxameia. As Peri fell victim to the final stages of the illness, The Doctor carried her in his arms through the desert back to the TARDIS. (The Caves of Androzani)
When the Great Intelligence invaded The Doctor’s time stream on Trenzalore to turn all of his victories into defeats, Androzani was one of the many places where The Doctor died. (The Name of the Doctor)