Simple, didn’t even think about this until now. The reason for this is that on the Key to Time: The Pirate Planet the Doctor and Romana go in search of the 2nd segment of the key to time. When the Doctor inserts the tracer into the TARDIS it comes back with the coordinates for the planet Calufrax. When they try and materialize the first time the TARDIS is nearly ripped apart for unknown reasons to the Doctor at the time. When they eventually materialize the following conversation takes place:
Romana: Shall we have a look at Calufrax now?
Doctor: Ah let’s get it over with. Horrible place. Cold, wet, icy. No life of any sort. Boring.
But when they look at the view screen they see that it is a nice weather outside. The Doctor is perplexed but realizes that the coordinates are indeed right and they set off to investigate. We fast forward to where the Doctor, Romana and Kimus have escaped from being prisoners of the Captain and are making their way to the mine shafts to take a look as the Doctor is curious about the mines. They enter a lift and proceed down. When they exit the lift they following conversation takes place:
Romana: Where are we Doctor?
Doctor: About 3 miles beneath the surface of Zanak I’d say.
Roman: 3 miles? But it’s so cold.
Doctor: Yes. And wet. And icy. Ah, Romana. This entire planet is hollow.
This is where the writers got the idea for the Donkey Wheel being beneath the Orchid station and why it is so very, very cold there. It also explains where they got the idea for the island moving when the wheel was turned. The entire planet of Zanak would move millions of light years from where it was and materialize around a slightly smaller planet and then it would mine it for everything that it had. The inhabitants of the planet were not aware of this other than the fact that the “lights in the sky would change” which the called the Omens because they were part of Zanak. This is where they got the idea of Richard and crew and others not jumping through time in my opinion because they were considered “part of the island.”