Off topics: Ringworld

Ringworld is a Hugo, Nebula, and Locus award-winning 1970 science fiction novel by Larry Niven, considered a classic of science fiction literature. I read this novel many years ago (around the 80's) and it is one of my favorite SF books along with its sequels: The Ringworld Engineers, The Ringworld Throne and Ringworld's Children.

The "Ringworld" is an artificial ring about one million miles wide and approximately the diameter of Earth's orbit (which makes it about 600 million miles in circumference), encircling a Sol-type star. It rotates, providing an artificial gravity that is 99.2% as strong as Earth's gravity through the action of centrifugal force. Ringworld has a habitable flat inner surface equivalent in area to approximately three million Earth-sized planets. The majority of the surface is land interspersed with shallow, freshwater seas. On opposite sides of the ring are two large deep saltwater oceans, placed in counterbalance to one another. Walls 1,000 miles tall along the edges retain the atmosphere.

Look at this wonderful animation:



More information at the Wikipedia