There are 270 Underground stations composing 11 lines in the London Underground, a metro system in the United Kingdom that serves Greater London and adjacent districts of Essex, Hertfordshire and Buckinghamshire. Its first section opened in 1863, making it the oldest underground metro system in the world. The system operates below ground in central London but generally runs on the surface in the outlying suburbs. Approximately 55 per cent of its routes are above ground. The system is operated by Transport for London (TfL). Most of the system is north of the River Thames with stations in the City of London or one of twenty-seven London boroughs. Stations at the north-eastern end of the Central line are in the Epping Forest district of Essex and stations at the north-western end of the Metropolitan line are in the Three Rivers and Watford districts of Hertfordshire and the Chiltern district of Buckinghamshire. (Full list...)