Houston is the fourth-largest city in the United States of America and the largest city within the state of Texas. As of the 2006 U.S. Census estimate, the city has a population of 2.2 million within an area of 600 square miles (1,600 km²). Houston is the seat of Harris County and the economic center of the Houston–Sugar Land–Baytown metropolitan area—the sixth-largest metropolitan area in the U.S. with a population of around 5.6 million. Houston's economy has a broad industrial base in the energy, manufacturing, aeronautics, transportation, and health care sectors; only New York City is home to more Fortune 500 headquarters. Commercially, Houston is ranked as a gamma world city, and the area is a leading center for building oilfield equipment. The Port of Houston ranks first in the United States in international waterborne tonnage handled and second in total cargo tonnage handled. The city has a multicultural population with a large and growing international community. It is home to many cultural institutions and exhibits—attracting more than 7 million visitors a year to the Houston Museum District. Houston has an active visual and performing arts scene in the Theater District and is one of few U.S. cities that offer year-round resident companies in all major performing arts.