I once read a sci-fi story about a distant future where mankind has recovered the earth with some kind of huge platform, massively increasing its habitable surface. To do this massive task they've been helped by an alien called something like Sissir or Sissyphe (I don't remember its exact name).
Below this platform there is some kind of jungle, with mutated animals and the humans who refused to live on the surface.
I can't remember the exact title or the author of that book, even though I quite liked it. I'd be grateful if someone knows it.