The X-Seed 4000 is a visionary concept for a megatall skyscraper. Its proposed height, sea-base, and 800-floor capacity could accommodate 500,000–1,000,000 inhabitants. This structure would have been composed of over of steel.
It was designed for Tokyo, Japan, by the Taisei Corporation in 1995 as a futuristic environment combining ultra-modern and technological living and interaction with wildlife and nature. Methods of transportation within the X-Seed would most likely include Maglev trains.
Georges Binder, managing director of Buildings & Data, a firm which compiles data banks on buildings worldwide, said the X-Seed 4000 "is never meant to be built[...] the purpose of the plan was to earn some recognition for the firm, and it worked."
Unlike conventional skyscrapers, to remain habitable the X-Seed 4000 would have been forced to actively protect its occupants from considerable internal air pressure and external air pressure gradations and weather fluctuations that its massive elevation would cause. Its design called for the use of solar power to maintain internal environmental conditions.