Ben Fry has created a stunning image consisting of the 26 million roads in the United States (click to zoom):
Nothing other than asphalt (gravel, dirt...) has been drawn here, but geographic and political features emerge nonetheless. In a very real sense, the geography is a latent feature of the roads dataset, as it creates boundary conditions for the observable effect (that being the roads themselves). In other words, we see mountains, rivers, oceans, and the Canadian border because they are defined by contiguous regions without any streets.
Please see Ben's project page for more information.