The Madrid Metro is one of the largest metro systems in the world, which is especially remarkable considering Madrid's population of approximately 3.5 million (Madrid city) to 6 million (metropolitan area). In fact, it is among the top 10 longest metros in the world, though Madrid is approximately the twentyfith most populous metropolitan area in the world. Taking into account a kilometer per inhabitant ratio, Madrid has the densest subway network in the world. It is also one of the fastest growing in the world, rivalled only by the Seoul Metro (Seoul); the latest round of expansions, completed in spring 2003, have increased its length to 226.7 kilometres. The metro has 190 stations on 12 lines (and one branch line.) An additional 44 km of metro lines are expected to be constructed by 2007, as well as 30 km of light rail lines that will serve the western region of the metropolitan area.
The metro opened on October 17, 1919 under the direction of the Compania de Metro Alfonso XIII. Metro stations served as air raid shelters during the Spanish Civil War. Starting in the 1970s, it was sequentially greatly expanded to cope with the influx of population and urban sprawl from Madrid's economic ascendancy. A huge project in the late 1990s and early 2000s installed approximately 50 kilometres of new metro tunnels, including a direct connection between downtown Madrid and Barajas International Airport (line 8), and service to outlying areas, including a huge 40-kilometre loop called MetroSur (line 12) serving Madrid's southern suburbs.