The Ringworld is an artificial world with a surface area three million times larger than Earth's, built in the shape of a giant ring orbiting its sun, a million miles across and with a diameter of 186 million miles. It was built by the Pak, who later through infighting left it mostly Protector free. It is inhabited by a number of different evolved hominid species, as well as Bandersnatchi, Martians and Kzinti.