London (/ˈlʌndən/ ( listen) LUN-dən) is the capital city of the United Kingdom. Standing on the River Thames in southeastern England, 50 miles (80 km) upstream from its estuary with the North Sea, London has been a major settlement for two millennia. Londinium was founded by the Romans. The City of London, London's ancient core − an area of just 1. 12 square miles (2. 9 km2) and colloquially known as the Square Mile − retains its medieval boundaries. [note 1] The City of Westminster is also an Inner London borough holding city status. Greater London is governed by the Mayor of London and the London Assembly. [note 2] London is a leading global city in the arts, commerce, education, entertainment, fashion, finance, healthcare, media, professional services, research and development, tourism and transportation. London, the fourth fastest-growing city, is 26 out of 300 major cities for economic performance. It is one of the largest financial centres and has either the fifth or sixth largest metropolitan area GDP. [note 3] It is the most-visited city as measured by international arrivals and has the busiest city airport system as measured by passenger traffic. It is the leading investment destination, hosting more international retailers and ultra high-net-worth individuals than any other city. London's universities form the largest concentration of higher education institutes in Europe. In 2012, London became the first city to have hosted three modern Summer Olympic Games.