The Radar: Underground Cities, Tech-Friendly Hotels, Most Expensive Airports in the U.S.

• Whether you find them creepy or cool, these eight underground cities and tunnels are worth a visit. [Matador]

• Discover five hotel brands and properties using digital technology to enhance the hospitality experience. [Overhead Bin]

• Huffington Post Travel rounds up the U.S.’s most expensive airports to fly out of, based on factors such as ticket value, taxes, and other fees. [Huffington Post]

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Photo: Dan Fenstermacher/My Shot