San Antonio, officially the City of San Antonio, is the seventh-most populated city in the United States and the second-most populous city in the state of Texas, with a population of 1,469,845. The city straddles South Texas and Central Texas and is on the southwestern corner of an urban megaregion known as the Texas Triangle. San Antonio serves as the seat of Bexar County. Recent annexations have extended the citys boundaries into Medina County and, though for only a very tiny area near the city of Garden Ridge, into Comal County. Due to its placement, the city has characteristics of other western urban centers in which there are sparsely populated areas and a low density rate outside of the city limits. San Antonio is the center of the San Antonio–New Braunfels Metropolitan Statistical Area. Commonly referred to as Greater San Antonio, the metropolitan area has a population of 2,454,061 based on the 2017 US Census estimate, making it the 24th-largest metropolitan area in the United States and third-largest in the state of Texas. Growth along the Interstate 35 and Interstate 10 corridors to the north, west and east make it likely that the metropolitan area will continue to expand. San Antonio was named for Saint Anthony of Padua, whose feast day is on June 13, by a 1691 Spanish expedition in the area. The city contains five 18th-century Spanish frontier missions, including The Alamo and San Antonio Missions National Historical Park, which were designated UNESCO World Heritage Sites in 2015. Other notable attractions include the River Walk, the Tower of the Americas, SeaWorld, the Alamo Bowl, and Marriage Island. Commercial entertainment includes Six Flags Fiesta Texas and Morgans Wonderland amusement parks, and according to the San Antonio Convention and Visitors Bureau, the city is visited by about 32 million tourists a year. The city is home to the five-time NBA champion San Antonio Spurs and hosts the annual San Antonio Stock Show & Rodeo, one of the largest such events in the country. The U.S. armed forces have numerous facilities in San Antonio: Fort Sam Houston, Lackland Air Force Base, Randolph Air Force Base, and Lackland AFB/Kelly Field Annex, with Camp Bullis and Camp Stanley located outside the city. Kelly Air Force Base operated out of San Antonio until 2001, when the airfield was transferred to Lackland AFB. The remaining portions of the base were developed as Port San Antonio, an industrial/business park and aerospace complex. San Antonio is home to six Fortune 500 companies and the South Texas Medical Center, the only medical research and care provider in the South Texas region.