London holds status of the world's most powerful, most coveted, most influential, most visited, most expensive, most sustainable, most investment-friendly, and most popular-for-work city. Arts, commerce, education, entertainment, fashion, finance, healthcare, journalism, professional services, research and development, tourism, and transportation are all affected. In terms of economic performance, London is ranked 26th out of 300 big cities. It is one of the world's most important financial centres, with the fifth- or sixth-largest metropolitan area GDP. It is the most visited city in terms of international arrivals, and its airport system is the busiest in terms of passenger flow. It is the most popular investment location, with the most international shops of any city. After Paris, London has the second-highest number of ultra-wealthy persons in Europe in 2017. Imperial College London in scientific and applied sciences, the London School of Economics in social sciences, and the comprehensive University College London and King's College London make up the biggest concentration of higher education institutes in Europe. London is home to a diverse population of people and cultures, with over 300 languages spoken in the area. Its estimated municipal population (equivalent to Greater London) in mid-2018 was 8,908,081, making it Europe's third most populous metropolis and accounting for 13.4% of the UK population. With 9,787,426 residents, London is Europe's fourth most populated city after Moscow, Istanbul, and Paris. The Tower of London, Kew Gardens, the Palace of Westminster, Westminster Abbey, and St Margaret's Church, and the historic settlement of Greenwich, where the Royal Observatory, Greenwich establishes the Prime Meridian (0° longitude) and Greenwich Mean Time, are all World Heritage Sites in London. Buckingham Palace, the London Eye, Piccadilly Circus, St Paul's Cathedral, Tower Bridge, Trafalgar Square, and The Shard are among the other notable sights. There are many museums, galleries, libraries, and athletic events in London. The British Museum, the National Gallery, the Natural History Museum, the Tate Modern, the British Library, and the West End theatres are among them. The London Underground is the world's oldest underground railway system.