World's tallest skyscraper would reach 2.5 miles high and house 1m residents

Dubai's Burj Khalifa currently holds the record for the world's tallest building, but a concept dreamt up three decades ago would dwarf it five times over should it ever become a reality.

First revealed in 1995, the design known as the X-Seed 4000 would comfortably claim the title of the most towering skyscraper ever built. It is projected to soar to a staggering 4km (or 13,123ft/2.5 miles) into the sky, placing it at nearly half the height of Mount Everest.

The Japanese construction giant behind the project, Taisei Corporation, reportedly envisaged it sitting atop Tokyo Bay, with other Asian locations also mooted as potential sites.

This extraordinarily bold undertaking proposed housing between 500,000 and one million residents across its 800 floors.

An estimated 3,000,000 tons of steel would have been required for the mammoth endeavour, with internal air pressure systems considered as a means of preventing altitude-related health issues amongst its inhabitants.

Recently debated on LinkedIn, the floating city, stretching 6km wide, was examined by Reuben Petty, who cast light on the vision that never came to pass.

He explained: "Its form was inspired by Mount Fuji, featuring a broad, stable base gradually tapering to a majestic peak. This pyramidal shape was not purely aesthetic; it was intended to provide structural stability against the forces of the ocean and potential seismic activity, a significant concern in the Tokyo Bay area."

The visionaries behind it imagined it would be a self-sustaining artificial island underpinned by floating caissons.

Far from merely consisting of residential units, the audacious proposal was for it to operate as a fully-fledged city in its own right, incorporating corporate centres, retail areas, entertainment venues and scientific institutions all accommodated on-site.

Rumours surfaced in 2007 suggesting Taisei intended to push ahead with the bold project, but these claims were swiftly refuted.

George Blinder, managing director of Buildings and Data, disclosed at the time: "It was never meant to be built. The purpose of the plan was to earn some recognition for the firm, and it worked."

Confirming the skyscraper was no longer being pursued, Shohei Ogawa, a planning manager at Taisei, said X-Seed 4000 was "on the shelf now".

He added: "It was our dream proposal for the technological advances we thought that could happen in the future."

Erik Howeler, an architect and author of Skyscraper: Vertical Now, told Architectural Record that building such a towering edifice was "possible" from an engineering standpoint.

However, from a practical angle, he felt the estimated $1 trillion cost would prove prohibitive, along with obtaining permission to construct such an enormous building. He added: "Tall buildings create a downdraft at their bases, and cast long shadows."

The scheme, which has triggered safety and environmental discussions, became a popular topic on Reddit. One user drew comparisons with a video game, stating: "This literally looks like the city seen in high charity mission in Halo 2."

Another likened it to a blockbuster film, commenting: "That's genuinely just the star harvester from Transformers 2."

A third raised concerns over practicality, saying: "Imagine the elevator not working and you live on the 657th floor."

Meanwhile, one person challenged the wisdom of the project, adding: "Ah yes, let's waste resources to build some massive building."