London is the capital city of England and the United Kingdom. It is the most populous city in the United Kingdom, with a metropolitan area of over 13 million inhabitants. Standing on the River Thames, London has been a major settlement for two millennia, its history going back to its founding by the Romans, who named it Londinium. London's ancient core, the City of London, largely retains its 2.9 km2 mediaeval boundaries and in 2011 had a resident population of 7,375; making it the smallest city in England. Since at least the 19th century, the term London has also referred to the metropolis developed around this core. The bulk of this conurbation forms the Greater London administrative area (coterminous with the London region), governed by the Mayor of London and the London Assembly. London is a leading global city, with strengths in the arts, commerce, education, entertainment, fashion, finance, healthcare, media, professional services, research and development, tourism, and transport. It is one of the world's leading financial centres and has the fifth-or sixth-largest metropolitan area GDP in the world depending on measurement. London is a world cultural capital. London contains four World Heritage Sites: the Tower of London; Kew Gardens; the site comprising the Palace of Westminster, Westminster Abbey, and St Margaret's Church; and the historic settlement of Greenwich (in which the Royal Observatory, Greenwich marks the Prime Meridian, 0° longitude, and GMT). Other famous landmarks include Buckingham Palace, the London Eye, Piccadilly Circus, St Paul's Cathedral, Tower Bridge, Trafalgar Square, and The Shard. London is home to numerous museums, galleries, libraries, sporting events and other cultural institutions, including the British Museum, National Gallery, Tate Modern, British Library and 40 West End theatres. The London Underground is the oldest underground railway network in the world.