X-Seed 4000 is a proposed skyscraper that looks oddly like Mt. Fuji. Perhaps it's unsurprising that it could be eventually built in Tokyo, Japan. The tallest building ever fully designed, the X-Seed 4000 would house between 500,000 and 1,000,000 people. A tiny quantity of individuals, if you take into account that the technology to "fabricate" mature adult individuals from raw materials is probably only a few decades away. (A topic that probably deserves a post all its own.)
The X-Seed 4000, which would be 4,000 meters high (13,123.2 feet), would in fact be taller than Mt. Fuji, which is merely 3776 meters high (12,388 ft).
The proposed structure would have a base 6 km wide and contain 800 floors. Designed by Taisei Construction Corporation, this mountain-shaped living environment would be powered by solar power and blend together high living with natural surroundings. It seems odd to claim that it would be powered by the sun entirely - this would mean that the company either intends to devote entire floors to nothing but solar panels or that it has found a way to power buildings economically with minimal paneling. In any case, it seems like nuclear power would be just as good.
Designed as an "intelligent building" the super-futuristically-named X-Seed 4000 would maintain light, temperature, and air pressure in response to changing external weather conditions. Because it has already been fully designed using materials available today, the structure could, in principle, be built, although it would likely cost several hundred billion dollars, if not more. Because the structure would weigh so much, it could only be built on the sea if present-day construction materials were used.
The X-Seed 4000 sounds an awful lot like an arcology, that is, ecological piece of architecture that would contain its own dependent ecosystem as well as human housing. In the long run, as buildings grow to such huge sizes that they become cities unto themselves, the integration of plants and animals will be essential for the preservation of human sanity and basic aesthetics. Eventually the Earth's surface could become entirely covered in such structures.