There's only so many place you can tweet from, and Twitter's been diligently indexing them all. These new visualizations from the Twitter team show the history of all geo-tagged tweets since 2009 (when the feature was born) by location. We're talking billions of them.
We've seen this sort of thing before, whether it's Twitter and Flickr usage across the country illustrated by bursts of light, or Foursquare check-ins displayed the same way. And hopefully we'll never stop seeing them, because they're always awesome.