Sequel to 'Ringworld' Ringworld: the most stunning artifact in known space, an artificial world with three million times Earth's surface area. Who built it? And where are they? Louis Wu (now a near-hopeless wirehead hooked on electrical ecstasy), the ageing 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 . . .