The Ringworld (Larry Niven, 1970)

In the year 2850, four explorers (two human and two aliens) explore a mysterious "ringworld": an enormous, artificial, ring-shaped structure that surrounds a star. When their ship crash lands on the Ringworld, the adventurers must set out to find a way to get back into space. They cross vast distances, witness strangely evolved ecosystems, and interact with some of the Ringworld's varied primitive civilizations. They attempt to discover what caused the Ringworld's inhabitants to lose their technology, and puzzle over who created the Ringworld and why.