Expo 75 was conceived, in part, to commemorate the American handover of Okinawa to Japan in 1972.

The centerpiece of Expo 75 was the Aquapolis a floating city designed by Japanese architect Kiyonori Kikutake.

It was envisioned as a concept of how humans could live harmoniously on the ocean, and a prototype for marine communities.

The Aquapolis was constructed at a shipyard in Hiroshima, Japan, and then towed to the Expo site.

Most of the exhibits were removed, although the Aquapolis was retained as an attraction and eventually hosted four million visitors.