Here's an interesting map...

...from Ezra Klein's piece in Vox today, "22 maps and charts that will surprise you."

This one shows that "Half of US GDP comes from these 23 orange blotches":

Half of America’s gross domestic product comes from the 23 orange blotches on this map. The other half comes from the vast acres of blue. Those orange blotches are America’s largest metro areas — and they absolutely power the American economy. But though small in size, they’re large in population: about 110 million people live in those orange blotches, says Reddit user Atrubetskoy, who created the map. So what you’re seeing on this map isn’t just that there’s a lot of economic activity compressed into some very small spaces in America, but that there are huge swaths of the American landscape where not that many people live.