I did not know that yesterday!

"The underground is beautiful in the way you can see the daylight like in the middle of the night" Benabar

The London Underground is an electric railway public transport network (a metro or subway system) that runs both above and below ground throughout the Greater London area. It is the oldest such transit system in the world. Operations began on 10 January 1863 on the Metropolitan Railway — the initial route is now part of the Hammersmith & City Line.

The London Underground is usually referred to as either simply "the Underground" by Londoners, or (more familiarly) as "the Tube", due to the shape of its deep-bore tunnels.

There are currently 274 stations open and over 253 miles (408 km) of active lines, as well as a number of stations and tunnels now closed. In 2004-05 total passenger journeys reached a record level of 976 million, or 2.67 million per day.

Since 2003, the Tube has been part of Transport for London (TfL), which also schedules and lets contracts for London's buses, including the famous red double-decker buses. Previously London Regional Transport was the holding company for London Underground.

History
For more details on this topic, see History of the London Underground. The first section of the London Underground (the "Metropolitan Railway", running between Paddington and Farringdon) was the world's first urban underground passenger-carrying railway. After delays for financial and other reasons following the scheme's adoption in 1854, public traffic eventually began on 10 January 1863. 40,000 passengers were carried over the line that day, with trains running every 10 minutes; by 1880 the expanded 'Met' was carrying 40 million passengers a year. Other lines swiftly followed, and by 1884 the Circle Line ("Inner Circle") was completed. All these early lines used steam-hauled trains, which required effective ventilation to the surface. Advances in electric traction later allowed tunnels to be placed deeper underground than the original cut-and-cover method, especially as deep-level tunnel design (including the use of tunnelling shields) improved. The first "deep-level" line, the City & South London Railway, now part of the Northern Line, opened in 1890.

In the early 20th century the presence of six independent operators running different Tube lines caused passengers substantial inconvenience; in many places passengers had to walk some distance above ground to change between lines. The costs associated with running such a system were also heavy, and as a result many companies
looked to financiers who could give them the money they needed to expand into the lucrative suburbs. The most prominent of these was Charles Yerkes, an American tycoon who in 1900–1902 acquired the Metropolitan District Railway and the Charing Cross, Euston & Hampstead Railway (to become part of the Northern Line) which has yet to be built, Great Northern & Strand Railway, Brompton & Piccadilly Circus railway (jointly to become the core of the Piccadilly Line) and Baker Street & Waterloo Railway (to become the Bakerloo Line), creating the Underground Electric Railways of London Company Ltd (Underground) on 9 April 1902. That company also owned many tram lines and proceeded to buy the London General Omnibus Company, creating an organisation colloquially known as the Combine.

In 1933, a public corporation called the London Passenger Transport Board was created. The Underground Group, the Metropolitan Railway and all the independent bus and tram lines were placed under the Board, an organisation which approximated in scope the current Transport for London. The outbreak of World War II, and especially The Blitz, led to the use of many Tube stations as air-raid shelters. Following the war, travel congestion continued to rise. The construction of the carefully planned Victoria Line on a diagonal northeast-southwest alignment beneath central London attracted much of the extra traffic caused by expansion after the war.

In 1977, the Piccadilly Line was extended to Heathrow Airport, and in 1979 the Jubilee Line was opened.