As of September 11, 2001, ground zero is now used in the United States of America to refer to the site of the World Trade Center after the terrorist attacks.

I'm not sure who first used the term to refer to the site, but it is evocative. Ground zero was also being used colloquially before the attacks to refer to any central location of any organization, or geographical area. Referring to the southern tip of Manhattan as "ground zero" refers as well to its place as the center of the financial world, the center of Manhattan, and in many ways, the center of many of the things that the United States stands for, like them or not, as well as describing not only the physical destruction of the Twin Towers and surrounding buildings, but the severe emotional damage the attack has had on the nation. Ground Zero also implies an emptiness, a sense of loss and displacement that is surely valid at this point, both physically and metaphysically.