Height comparison between the X-Seed 4000 and the Burj Khalifa The X-Seed 4000 was a concept for a megatall skyscraper.[1] Its proposed 4-kilometre (2.5 mi) height, 6-kilometre-wide (3.7 mi) sea-base, and 800-floor capacity could accommodate 500,000-1,000,000 inhabitants. This structure would have been composed of over 3,000,000 tons of steel. It was designed for Tokyo, Japan, by the Taisei Corpor