The London Underground

London Underground

The London Underground is a rapid transit system serving a large part of Greater London and some parts of Buckinghamshire, Hertfordshire and Essex in England...

 is a metro system

Rapid transit

A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...

 in the United Kingdom that serves Greater London

Greater London

Greater London is the top-level administrative division of England covering London. It was created in 1965 and spans the City of London, including Middle Temple and Inner Temple, and the 32 London boroughs. This territory is coterminate with the London Government Office Region and the London...

 and adjacent districts of Essex

Essex

Essex is a ceremonial and non-metropolitan county in the East region of England, and one of the home counties. It is located to the northeast of Greater London. It borders with Cambridgeshire and Suffolk to the north, Hertfordshire to the west, Kent to the South and London to the south west...

, Hertfordshire

Hertfordshire

Hertfordshire is a ceremonial and non-metropolitan county in the East region of England. The county town is Hertford.The county is one of the Home Counties and lies inland, bordered by Greater London , Buckinghamshire , Bedfordshire , Cambridgeshire and...

 and Buckinghamshire

Buckinghamshire

Buckinghamshire is a ceremonial and non-metropolitan home county in South East England. The county town is Aylesbury, the largest town in the ceremonial county is Milton Keynes and largest town in the non-metropolitan county is High Wycombe....

. 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 comprises 11 lines, serving 270 Underground stations.