A megacity is defined an metropolitan area in excess of 10 million people. That's no problem in Tokyo, I mean, there are that many people in the subway right now. But check this out:So, this is the view of the #1 megacity from the 45th floor of the Tokyo Metropolitan Building in the hood of Shinjuku... on a cloudy day.No mater how far you look, all you see is an urban landscape. Even in Chicago, it's not far till you see Wisconsin or Rock Island.
Megacity #2 and #3 is New York and Mexico City.