Can it be a city floating in the sky? If you haven't heard of the concept, let me introduce you to the late, great, Buckminster Fuller's Cloud 9 concept.

Geodesic domes are one of the few structural forms that gets proportionally stronger the larger it gets. Which means that, in principle, you can build geodesic domes of massive size. You could in principle build geodesic domes across entire metropolitan areas if you wanted to.

But an interesting thing happens if you make them big enough, as Buckminster Fuller realized. If you have a geodesic sphere a half a mile in diameter, the air inside it weighs about thousand times more than the geodesic sphere enclosing that air.

And this means, if you heat the air in such a sphere the slightest bit warmer than the air inside, that sphere will experience a colossal buoyant force. You could build a city in such a sphere and it would heat itself. Just the body heat of the people living there plus the heat output of all their machines and the heat used to heat and cool homes would be more than enough to raise the temperature of the entire sphere to the point where the city could float.

In other words, such a city would be a giant hot air balloon; except it wouldn't even need a burner, the waste heat of its inhabitants would keep it constantly warm enough to make the whole city float.

I love this concept because flying cities are one of those things casually thrown into a scifi setting to indicate an impossibly advanced civilization. Just throw a floating city on screen, and you know that civilization must have some sort of anti-gravity technology. They're capable of doing things that are so far beyond us they might as well be magic. We have no more idea how to create anti-gravity than we know how to be a literal magical wizard.

But it turns out, we actually could make flying cities today if we wanted. We don't have anti-gravity, but if enough raw buoyant force, you can lift an entire city. If we wanted to, we could have a world where we look up at the sky and see an entire city lazily bobbing about, slowly drifting on the wind, gradually passing over you and floating off over the horizon.