A two-kilometre-tall pyramid proposed for Tokyo Bay — the single largest structure ever conceived by a real engineering firm, designed to house 750,000 people.

Proposed by Shimizu Corporation in 2004, this megastructure would be 12 times taller than the Great Pyramid of Giza. Current materials cannot support the weight — the design requires carbon nanotubes that don't yet exist at scale. It remains the most ambitious architectural concept ever seriously engineered.