The X-Seed 4000 is the tallest building ever fully envisioned, meaning that the designs for construction have been completed.
The X-Seed 4000 "was 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."
Its proposed 4 kilometer (13,123 foot) height, 6 kilometer wide sea-base, and 800 floor capacity could accommodate five hundred thousand to one million inhabitants.
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. Designed as an “intelligent building,” the futuristically-named X-Seed 4000 would maintain light, temperature, and air pressure in response to changing external weather conditions. Its design calls for the use of solar power to maintain internal environmental conditions.
A sea-based location and a Mount Fuji shape are some of this building's other major design features — the real Mount Fuji is land-based and is 3.8 kilometers in height, nearly 213 m (700 feet) shorter than X-Seed 4000.
Some estimate that the cost to construct the X-Seed 4000 structure may be somewhere between US$300-900 billion, in current dollars.
(Information comes courtesy of the Wikipedia amongst other sources.)