Next year I will be going to college for civil engineering and I can't wait. One of the people in my physics class is always asking me if I'm going to build an Atlantis. Ever since, I have been interested in the idea of building a floating city on water, called seasteading, which is what Atlantis was before it sank supposedly. Current engineers are looking into the idea, using a similar set up as oil rigs, which are held up on platforms that go down to the bottom. The issue is the cost and the politics of it.
There are floating bridges in existence, which use multiple air compartments so that in the case of a leak, the bridge will not collapse. However, these bridges are mostly used temporarily because it is not always the most stable or reliable. Thus, this same technology would not be possible. Not to mention that a car is a whole lot lighter than a city.
So are floating cities a possibility? I don't know, but I want to try. Even if it means taking all the floaties in the world and building a city on top. Think it will float?