Arlington is a city in Tarrant County, Texas (USA) within the Dallas–Fort Worth–Arlington metropolitan area. According to the 2010 census results, the city had an estimated population of 365,438, making it the third largest municipality in the Metroplex. Arlington is the seventh-largest city in Texas and the 50th largest city in the United States.Located approximately 12 miles (19 km) east of downtown Fort Worth and 20 miles (32 km) west of downtown Dallas, Arlington is home to the University of Texas at Arlington, the Texas Rangers' Ballpark in Arlington, Cowboys Stadium, the International Bowling Campus (which houses the United States Bowling Congress, International Bowling Museum and the International Bowling Hall of Fame), the headquarters for American Mensa, and the theme parks Six Flags Over Texas (the original Six Flags) and Hurricane Harbor. The city borders Kennedale, Grand Prairie, Mansfield and Fort Worth, and surrounds the smaller communities of Dalworthington Gardens and Pantego. Arlington is the headquarters of the Nuclear Regulatory Commission Region IV.Arlington is the largest city in Texas to be contained within only one county; it does not extend city boundaries to another county. Likewise it is also the largest city in the state that is not a county seat.