SOLVED. Sci fi novel; primitive tribes, jungle, spaceship, tunnels. [s]

[A book away from an episode of hoarders]

So we were watching Snowpiercer when my dad said it reminded him of a book he read "a few years ago". My father is in his seventies and has said this could be pre-1970.

He said that apparently the sun was going to burn out so the whole world got together and build this massive spaceship to hold mankind so that we could survive until the ship found a habitable planet where we could stay.

There were primitive tribes and the whole ship had become overgrown into jungles because of some issue with the greenhouses. The main characters found a section of the ship that had no jungle and apparently that's when we realize they'd been on a spaceship all along.

There was a tunnel, and you find out the ship landed itself a while back and they were on a planet.

[A book away from an episode of hoarders]

I should clarify; the people didn’t start out as primitive. They were modern when they boarded the spaceship. No one, during the story, had any idea that they were on a ship. It was just an overgrown jungle. He said he thought that the ship had left earth a LONG time before the story begins, possibly thousands of years.

And that the ship was programmed to find the habitable planet, and land, all on its own, which apparently it did, possibly even before the book’s beginning. It just didn’t let the people out for whatever reason.

[Stephen]

Brian Aldiss's Non-Stop matches the middle part: primitive tribes, overgrown vegetation, discovery that they're really on a starship.

But I don't think it matches the beginning and end. IIRC, the passengers were returning to Earth from a space colony, and in the end it turned out that the ship had arrived at Earth, but hadn't "landed": it was quarantined in orbit.

[A book away from an episode of hoarders]

I'll check with him, thanks. It's possible some details got mixed up over the years. We read a lot. :)

[A book away from an episode of hoarders]

I got a copy of Nonstop, also known as Starship, for my father. This was the book he was thinking of. Thanks for all your help. :)

[Stephen]

You're welcome! Glad it matched.