X-Seed 4000
Taisei Corporation drew a four-kilometre mountain for Tokyo Bay, a building so tall that its summit would rise far above Mount Fuji.
Quick facts
- Existence state
- Never Born
- Peak stage
- documented concept
- Research confidence
- high
- Last verified
What it was
A conceptual supertall arcology attributed to Japan's Taisei Corporation.
The stepped conical structure was presented as a self-contained vertical city for hundreds of thousands of residents, powered in part by solar energy.
Existence timeline
Taisei presents a four-kilometre vertical city for Tokyo.
The project uses an open mountain-like frame around inhabited districts.
The scheme imagines housing hundreds of thousands of people.
The extreme concept circulates through technical and popular publications.
No client, site approval or construction programme follows.
Tall-building and science archives continue to catalogue the proposal.
Why it ended
X-Seed 4000 was a publicity and research concept rather than a commissioned development, and never entered detailed construction planning.
Last confirmed trace
Corporate illustrations, engineering references and tall-building databases preserve the deliberately extreme proposal.
What it left behind
It remains a recurring benchmark for the difference between a drawable skyscraper and a project with any realistic route to the ground.
How we classified it
The proposed archive unit never reached ordinary public access in the form being documented.
The strongest verified milestone maps to documented_concept under Arrival Score V1.
Sources & verification
Last reviewed 2026-08-01. Sources are linked so readers can inspect the evidence directly.