Journalist and author Doug Saunders has spent years studying what he calls "arrival cities" -- the ramshackle neighborhoods on the outskirts of the world's biggest cities that are often the place new immigrants land. Arrival cities are often kept down by physical and bureaucratic barriers, he says, but their residents' drive to succeed makes the neighborhoods valuable to the urban area at large. Saunders visits Zócalo to discuss the arrival cities -- including those in Los Angeles, Istanbul, Cairo and many other urban centers and the challenges and opportunities they pose.