The Ringworld engineers

Ringworld: the most stunning artifact in known space, an articficial world with three million times Earth's surface area. Who built it? And where are they? In this stunning sequel to Larry Niven's Hugo and Nebula award-winning novel, Louis Wu (now a near-hopeless lirehead hooked on electrical ecstasy), the aged Kzin warrior, Speaker-to-Animals, and the Hindmost, puppeteer mate of mad Nessus, return to Ringworld. Their aim is to prevent cataclysm. IN the process, they find themselves learning Ringworld's incredible secrets...