In 1995, Japanese construction firm Taisei Corporation unveiled a concept for the X-Seed 4000, a skyscraper that would have soared 4km (13,123ft) into the sky—five times the height of the Burj Khalifa. Designed to float on Tokyo Bay, the structure would have housed up to one million people across 800 floors.
The pyramid-shaped design, inspired by Mount Fuji, aimed to provide stability against seismic activity and ocean forces. The project would have required 3 million tons of steel and featured internal air pressure controls to prevent altitude sickness. The floating metropolis would have spanned 6km wide, functioning as a self-contained city with business centres, residential areas, and recreational facilities.
Despite its ambition, the X-Seed 4000 was never intended for construction. George Blinder, managing director of Buildings & Data, stated in 2007: "It was never meant to be built. The purpose of the plan was to earn some recognition for the firm." Shohei Ogawa, a planning manager at Taisei, confirmed the project is "on the shelf now," calling it a "dream proposal for technological advances."
Architect Erik Howeler noted that while the building was "possible" from an engineering standpoint, the estimated $1 trillion cost and challenges like downdrafts and shadows made it unfeasible. The concept has since sparked online discussion, with some comparing it to fictional structures from video games and films.
