Empress of the Sun

The airship Everness makes a Heisenberg Jump to an alternate Earth unlike any her crew has ever seen. Everett, Sen, and the crew find themselves above a plain that goes on forever in every direction without any horizon. They've arrived on an Alderson Disc, an astronomical megastructure of incredibly strong material reaching from the orbit of Mercury to the orbit of Jupiter.

Who could have built such a thing? The Jiju, the dominant species on a plane where the dinosaurs didn't die out. They e...