What book has Earth's surface covered with a raised platform with a jungle underneath?

Question

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.

Thanks!

Answer 1

I'm quite sure this is "The Realms of Tartarus." It's been a long time since I read it, but it does have a utopian world built on a platform above the earth's surface with a left-over ecology below, where life evolves in strange ways.

I didn't recall it having any aliens, but according to this page there is an alien named "Sisyr" involved, so that fits your recollection. That same page says that "Realms" is a trilogy made up of 3 short novels, "The Face of Heaven," "A Vision of Hell," and "A Glimpse of Infinity."