London is one of the leading global cities in the arts, commerce, education, entertainment, fashion, finance, healthcare, media, professional services, research and development, tourism and transportation. It is the world’s largest financial centre and has the fifth or sixth largest metropolitan area GDP in the world. London is often regarded as a world cultural capital. It is the world’s most-visited city as measured by international arrivals and has the world’s largest city airport system measured by passenger traffic. It is the world’s 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 the modern Summer Olympic Games three times. 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 defines the Prime Meridian, 0° longitude, and GMT). Other 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, Natural History Museum, Tate Modern, British Library and West End theatres. The London Underground is the oldest underground railway network in the world.