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A Heligan Structure was a space station which was also a tree.
Humans tried to terraform the planet Golrandonvar so they could colonise it. This would have killed the native Thara, as the methane they needed would have been eliminated. The Eleventh Doctor helped the natives to fight back, and the humans were stranded on a Heligan Structure. The Fourth Doctor and Leela visited the Structure centuries later, unaware of the events caused by his later incarnation. (PROSE: The Roots of Evil)