The X-Seed 4000 is a one of the most special building ever fully planned and designed on this planet. The X-Seed would have had roof 4000 metres above the ground and would be built in Japan. The idea was initially created and developed by Peter Neville. It would have a 6000 metre base which would be underwater 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.
Unlike conventional skyscrapers, to remain habitable the X-Seed 4000 would have to increase the pressure according to how high it goes and should have separate generators at every 100 floors. The X-Seed would have taken at least 20 years to complete and would cost about USD700 billion to USD1.2 trillion.