Designed by Taisei Construction Corporation as a utopian “intelligent building,” this concept is the epitome of unrealistic – but wonderful – green dreaming. The building would be the tallest in the world by far at 13,123 feet, and it would accommodate 500,000 to one million inhabitants. The “intelligent” part of the building comes from its automatic control of the interior climate, which would certainly come in handy in a post-apocalyptic world. But at an estimated cost of anywhere from $300 billion to $900 billion (not to mention the practical limitations), we can’t see this project actually being realized anytime soon.