What is the setting of The Lost Hero by Rick Riordan?

The story is set in the USA, in which the characters travel to many different destinations. The USA is the current home of the Greek gods, as this is where Western civilization is strongest. The entrance to Mount Olympus is now located inside the Empire State building in New York. This setting allows for ancient Greek mythology to be presented with a modern, American twist, in a setting that is more familiar to the book's readers.