I have been traveling my entire life. As a journalist, I've reported from places most people never see, like small towns in Malaysia and factory complexes in Tijuana. As a traveler, I've chased the unusual, the overlooked, the wonderful, and the natural around the world. I run marathons and rock climb, so discomfort in the name of discovery is basically my love language. And yet, until this past December, I had never once asked myself a simple question: how many of the fifty United States have I...