The X-Seed 4000 is the tallest building ever fully envisioned. Its proposed 4000m (2.485 miles) height, 6 km (3.728 miles) wide sea-base, and 800 floor capacity could accommodate five hundred thousand to one million inhabitants.

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It was designed for Tokyo, Japan by the Taisei Corporation in 1995 as a futuristic environment combining ultra-modern living and interaction with nature.

Unlike conventional skyscrapers, the X-Seed 4000 would be required to actively protect its occupants from considerable air pressure gradations and weather fluctuations along its massive elevation. Its design calls for the use of solar power to maintain internal environmental conditions.

"The X-Seed 4000 is never meant to be built," says Georges Binder, managing director of Buildings & Data, a firm which compiles data banks on buildings worldwide. "The purpose of the plan was to earn some recognition for the firm, and it worked."