London's famous Underground system is the oldest of its type in the world - the first services started running in the 1860's - and is known affectionately as 'The Tube' by locals and visitors alike.
Contrary to its title, a majority of the network actually lies above ground level, in fact most of the underground parts of the Tube are in central London where overland tracks would use up valuable real estate.
The network itself runs for a total of 253 miles, serving nearly three hundred stations across Greater London and beyond, and on any given day over two and a half million journeys are made by passengers.
The now extensive Underground system came from humble beginnings with just a single section of line running between Farringdon and Paddington at its opening, but it was to become hugely popular virtually overnight and, by the turn of the century, no less than six independent companies were operating their own lines.
In the first half of the 20th century new lines sprang up across London, creating much of the tunnel network we see today, and even now the system is constantly being refurbished and extended.
The Tube network is made up of a number of different lines each of which is conveniently colour-coded - for instance, the Central Line that crosses central London linking the East and West Ends is coloured red. This colour-coding enables passengers unused to the Underground to follow the Tube map (which has, incidentally, become something of a design classic) and plan their journey.
Two events in recent times have had profound effects on both the Tube itself and its image: the 1987 Kings Cross fire, which killed 31 people at one of the city's busiest stations and was started by a discarded cigarette, led to a full smoking ban across the network. The 7 July attacks, which involved bombs on three tube trains and a double decker bus, killed 51 passengers and injured many more.