Ringworld

Imagine an enormous ring around a star, encircling it at a distance roughly the same as Earth is from the Sun. Such a structure would have many times the amount of living space that a planet could contain. The Piersen's puppeteers, an alien race of congenital cowards, have found just such a structure orbiting a distant star. They want to know who built it and what significance it may hold for them. And they have picked the human Louis Wu to head up the team that will investigate. The mission seems simple enough, until the Ringworld's automatic meteor defenses damage their spaceship. Suddenly they are stranded on the Ringworld, amidst the wreckage of a long-dead civilization and its regressing survivors. Now they must penetrate the Ringworld's secrets if they are to get back off and return home with their discoveries.