Ringworld

The Ringworld or Niven ring is an artificial world in the Ringworld Universe with a surface area three million times larger than Earth's, built in the shape of a giant ring-shaped ribbon a million miles wide and with a diameter of 186 million miles, encircling a Sun-type star. It rotates, providing an artificial gravity equivalent to 99.2% of Earth's gravity by way of Centrifugal force.

Ringworld has a habitable flat inner surface equivalent in area to approximately three million Earth-sized planets. It's surrounded by a massive sunshade to protect the inhabitants from the star's radiation. 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 1000 miles tall along the edges retain the atmosphere. The Ringworld could be regarded as a thin, rotating slice of a Dyson sphere.