The H Street Wal-Mart in Washington, D.C. Ten years ago, none of the city's 600,000 residents lived within 1 mile of a Wal-Mart. Today, almost 13 percent do. Emily Jan/NPR hide caption

itoggle caption Emily Jan/NPR

The Los Angeles River in 2013. Engineers turned it into a narrow concrete channel in the 1940s, after a flood destroyed homes and left 100 people dead in 1938. Steve Lyon/Flickr hide caption

itoggle caption Steve Lyon/Flickr

Zappos.com CEO Tony Hsieh is spearheading an effort to revitalize downtown Las Vegas and make it a tech hub, home for small businesses and a creative community. Isaac Brekken for NPR hide caption

itoggle caption Isaac Brekken for NPR

A view of Seattle from the Bullitt Center. Brad Kahn/Flickr hide caption

itoggle caption Brad Kahn/Flickr

Kevin Faulconer reaches for his wife, Katherine, at a Feb. 11 rally as their daughter looks up from below. Faulconer, a Republican, won the San Diego mayoral race last week. Lenny Ignelzi/AP hide caption

itoggle caption Lenny Ignelzi/AP

Sadia Bies and Boston Mayor Thomas Menino stand next to her portrait of him at a July press conference. Menino, the city's longest-serving mayor, is stepping down at the end of his term. Aynsley Floyd/Invision for Gillette hide caption

itoggle caption Aynsley Floyd/Invision for Gillette

Political theorist and author Benjamin R. Barber (left) spoke at the CityLab summit this week in New York. He is proposing the formation of a "World Parliament of Mayors." Courtesy of The Atlantic hide caption

itoggle caption Courtesy of The Atlantic

World War Z is just the latest pop-culture incarnation of the Zombie Apocalypse. Adam Frank says the zombies keep coming because they're trying to tell us something. MPC/Paramount Pictures hide caption

itoggle caption MPC/Paramount Pictures

Information, like light, flows through cities in interesting, often beautiful, patterns. Valery Hache/AFP/Getty Images hide caption

itoggle caption Valery Hache/AFP/Getty Images

Adam Frank stands atop of the Wilder Building in Rochester, N.Y. Carlet Cleare /WXXI hide caption

itoggle caption Carlet Cleare /WXXI