In the tenth part of our series on the drivers of the world's busiest airports, Denver International provides some unique features. For starters, the new Denver International Airport (DIA) is the only major green-field airport opened in the US in the past two decades. Equally unusual, when the new field was opened in 1995, one of its long-time airline tenants, Continental, stopped using the airport as a hub. And over time, the new airport has become a hub for a legacy carrier, United, as well as for two low cost carriers, Frontier and Southwest.