London is the capital city of the United Kingdom and largest metropolitan area in Europe. The London Eye is the tallest Ferris wheel in Europe, visited by over 3.5 million people annually. Big Ben refers to the great bell of the clock at the Palace of Westminster and the clock tower, which contains the largest four-faced chiming clock and third-tallest free-standing clock tower in the world. London Bridge spans the River Thames, connecting the City of London and Southwark.