It starts in an ordinary small town. Suddenly it turns out that some advanced civilization has combined many different worlds into one huge sphere. People can look inside it, and in the center there is something like a sun that provides light.
Nearby there are hostile creatures — kind of insect-like, maybe ant-like — that try to steal all the metal. The main characters defend themselves from them using automatic rifles. Early in the story there is a battle over railroad tracks that the creatures are trying to steal.
Later, the characters travel far across this artificial world, exploring it and gradually understanding what it really is.
by Famous_Geologist2297