The X-Seed 4000 is a visionary skyscraper for what would be, if it was built, the tallest building in the world. The idea was initially created and developed by Peter Neville. Its proposed 4-kilometre (2.5 mi) height, 6-kilometre-wide (3.7 mi) sea-base, and 800-floor capacity could accommodate 500,000 to 1,000,000 inhabitants. This structure would be composed of over 3,000,000 tons of pure steel.
It was designed for Tokyo, Japan by the Taisei Corporation in 1995 as a futuristic environment combining ultra-modern living and interaction with 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 & 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."
Unlike conventional skyscrapers, to remain habitable the (X-Seed 4000) would be 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 calls 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 be subject to earthquakes and tsunamis.
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 3,776 metres (12,388 ft) high so is 224 metres (735 ft) shorter than the X-Seed 4000.
The X-Seed 4000 is projected to be twice the height of the Shimizu Mega-City Pyramid at 2,004 metres (6,575 ft). The Shimizu Mega-City Pyramid (proposed in 2007, also planned for Tokyo, Japan) faces most of the same problems as the X-Seed. Other projects that may be in the top five man made structures are the Ultima Tower 3,218 metres (10,558 ft) in San Francisco, Dubai City Tower 2,400 metres (7,900 ft) and the Bionic Tower 1,228 metres (4,029 ft) in either Hong Kong or Shanghai.