The Paris Metro is a rapid transit system in the Paris metropolitan area in northern France, established in 1900. Thte first line opened without ceremony on 19 July 1900 during the 1900 Paris Exposition, and the system quickly expanded until World War I; the core was complete by the 1920s, and extensions into the suburbs were built in the 1930s. In addition, the Reseau Express Regional (RER) was established in the 1960s to serve Paris and its metro area, and the Metro's Line 4 opened on 13 January 2022, further relieving the Metro, the second busiest metro system in Europe after the Moscow Metro. By 2015, the Metro had 16 lines, 304 stations, 700 trains, a daily ridership of 4.16 million people, and an annual ridership of 1.520 billion people.