KEID: A Lost Civilization by Robert G. Williscroft

They discover a Dyson Sphere under construction by microbots, but the bots are scavenging one part of the sphere to construct another. And where is the civilization that designed this massive project?

“This is cutting edge, hard science fiction at its finest. This broad-minded, far-sweeping saga is both realistic and daring. The reader feels that yes, this is what the universe is like and what man’s role in it will come to be.” Professor John B. Rosenman, Norfolk State University.

The starship Andromeda stops at the Keid triple-star system on its voyage of discovery and stumbles on a derelict Dyson Sphere. The starship is attacked by microbots scavenging one part of the sphere to repair another. They map the sphere and discover several primitive races inside—humanoid, feline, and saurian. They investigate the internal structure of the Dyson Sphere with its hundred-kilometer wide tubes filled with fast-moving magnetic fluid, and I-beams with hundred- kilometer wide plates and thirty-kilometer wide flanges that are one hundred meters thick. They eventually meet digital uploads of the race that built the sphere.

Then the explorers discover that the Dyson Sphere’s star is about to produce a superflare that could destroy all life inside the sphere and possibly even the Dyson Sphere itself. The explorers mount a major rescue effort to save the uploads and sapient races.