In 1995 the Japanese construction giant Taisei Corporation unveiled plans for the X-Seed 4000, a tower four kilometers tall — a building shaped like Mount Fuji that would have stood 224 meters higher than the actual mountain.
It called for 800 floors, room for up to a million residents in Tokyo Bay, and more than three million tonnes of steel. The upper floors sit so high in the atmosphere that the design needed active pressurization to keep the air breathable near the top.
From Wikipedia:
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. As the proposed site for the structure is located in the Pacific Ring of Fire, the most active volcano range in the world, the X-Seed 4000 would have been vulnerable to earthquakes and tsunamis.
Sadly, nobody ever intended to build it. As the architecture writer Georges Binder put it, the X-Seed "is never meant to be built," and "the purpose of the plan was to earn some recognition for the firm, and it worked." Thirty years later, the Burj Khalifa, the tallest building actually standing on Earth, still reaches less than a fifth of the X-Seed's paper height.
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