Okay, that headline might be ever so slightly hyperbolic, but there are underwater cities you can build in Anno 2070. Maybe we can get a user-made mod to bring the concept of BioShock's Rapture in this game closer to reality. Because seriously, that would rule.
Apparently, rising sea levels will one day push humanity into the ocean depths; at least, that's the backdrop for Anno 2070. Truth be told, I don't need a premise to get worked up about managing cities under the sea. Darling it's better down where it's wetter, take it from me.