The London Underground is an iconic underground metropolitan railroad located in London, England. In addition to shaping the city of London above it, the London Underground also influenced metropolitan railroads around the world. Many nations modeled their subway systems after the London Underground. The London Underground is also known as the Tube, due to the characteristic shape of the subway tunnels.

The total network which comprises the London Underground is 253 miles (408 kilometers) long. 976 million rides are taking on the London Underground each year in 4,070 subway cars. The London Underground began in 1863, when service was first opened between Paddington Station and Farringdon Street. The system has continued to expand, with periodic constructions of new lines and retrofitting of old ones to better serve the users of the London Underground.

Construction on the metropolitan railway system, which came to be known as the London Underground, began in the Victorian era in response to increasing traffic congestion. The growing population made traveling in London very difficult, and it was thought that an underground railway would solve many congestion problems. The London Underground expanded rapidly, and included 12 separate lines by 2006.

Initially the lines were constructed using a cut and cover technique. This involved completely excavating the dirt, building a supported brick arch, and covering the arch. This technique was time consuming and contributed to congestion as well, as it blocked off vital roadways. Later, a system similar to that used in coal mining was used, where tunnels were excavated and supported with beams before being brick lined.

The first line to be completed was the Circle Line, which was finished in 1884. Other lines followed quickly, including the Central Line, Bakerloo, and Hammersmith and City. Initially the London Underground used steam trains, but began to convert to electric trains in the early 1900s. The London Underground introduced electronic ticket machines in 1918, and continued to be at the forefront of metropolitan railway innovation.

The London Underground's extensive network was employed for use as underground bomb shelters during the Second World War. Britons of all ages and classes sheltered underground during numerous bombings of London. Expansion on the London Underground ceased during the war, with partially completed lines being used for storage and basic manufacturing. In 2003, the London Underground was integrated into the Transport for London system, which is aimed at increasing efficiency and capability of London's transit services.

The London Underground also utilizes a unique map, designed by Harry Beck in 1933. Beck realized that a geographically accurate and properly scaled map would be difficult for travelers to read. His map simplified the London Underground to its essence, color coding the various lines and smoothing out their routes so that the entire system could be represented simply and plainly. Maps styled after his are used for transit systems everywhere to quickly convey basic information about routes to travelers.

The London Underground also has several “ghost stations,” Tube stations which are no longer used although they still exist. These disused stations were closed for various reasons. Some of them are like time capsules perfectly capturing the moment at which they were closed, while others have been more or less erased from history. Tours of the disused stations are sometimes held, and are an interesting look into the heart of the subway system.