X-Seed 4000

X-Seed 4000 is a visionary megatall skyscraper.
Size.
Its proposed 4 km height, 6 km wide sea-base, and 800 floors capacity that could fit 500,000 to 1,000,000 people. This structure would have been made using 3 billion kilograms of steel.
Design.
It was designed for Tokyo, Japan. The design was made by the Taisei Corporation in 1995. It was designed as a future-like 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.
"The X-Seed 4000 'is never meant to be built'." says Georges Binder, managing director of Buildings &amp; Data, a firm which compiles data banks on buildings worldwide. "The purpose of the plan was to earn some identification for the firm, and it worked."
Air pressure.
Unlike other skyscrapers, the X-Seed 4000 would be forced to protect its occupants from large internal/external air pressure effects and weather rising that its great elevation would cause. Its design calls for the use of solar power to keep internal environmental conditions. 
Risks.
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 be weak to earthquakes and tsunamis.
Height comparisons.
A sea-based location and a Mount Fuji shape is some of this building's other major design features. The real Mount Fuji is land-based and is 3,776 metres in height. This is 224 metres shorter than the X-Seed 4000.
X-Seed 4000 is planned to be twice the height of the Shimizu Mega-City Pyramid at 2,004 metres. 
The Shimizu Mega-City Pyramid for Tokyo was proposed in 2007. This plan also faces most of the same problems as X-Seed 4000.
Others that may be in the top 5 man-made structures are: