I still remeber that one episode when the main character goes up a stairwell for quite a while (possibly weeks+) then finally reaches the top, opens a manhole cover basically, enetering a huge room - for one he can't see the ceiling and it goes in all directions and seems flat.

After wandering around for an unspecified while he encounters a cyborg scientist looking through some sort of instrument. He tells him this "room" in the megastructure is a spehrical empty space and has the inner diameter of Jupiter.

So if you ever wondered where to Von Neumann construction bots that got a little haywire got all the raw materials...