Projected Population in 2015: 19.9 million
Population in 2005: 18.7 million

The New York metropolitan area constituted the world's first megacity, passing the 10 million mark by 1950. It was founded during the 1620s as New Amsterdam, a Dutch colony that soon passed to British control. The key to its 19th-century growth was the Erie Canal, completed in 1825, which funneled commerce from America's interior to the Hudson River. These days, New York no longer is the world's busiest port, but it remains America's finance and media capital.