The X-Seed 4000 is the second tallest building ever fully envisioned (after the Tokyo Tower of Babel), meaning that the designs for construction have been completed. The idea was initially created and developed by Peter Neville. Its proposed height, wide 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 reinforced steel.