Ringworld

The Ringworld is a vast, ring-shaped artificial structure. With over 186 million miles in diameter (~ 300 million km) and a million miles of width (~ 1.6 million km), its surface area is roughly equivalent to three million times Earth's, and its rotation provides an equivalent of gravity by means of centrifugal force. The inner atmosphere is contained by rim walls about 1,000 miles high (~ 1,600 km).

The result is that the inner surface of the Ringworld contains a habitable environment for Humans and numerous other alien races. An artificial day-and-night cycle is provided by an inner ring of interconnected shadow squares which periodically block the sunlight.

The Ringworld is the only body in orbit around its primary star, and speculated to have been constructed by cannibalizing the star's natural planetary system, countless ages ago, to provide energy and perhaps raw material. The material that the Ringworld is made of, however, is called scrith: a substance that provides a tensile strength comparable to the strong nuclear force of atoms; making the whole thing surprisingly stable and durable.