Trains have travelled beneath the streets of London for more than a century, becoming a vital part of life in the busy capital.
The London Underground system, which first began 154 years ago, has for most of its continually evolving history been known simply as "the Tube".
But the affectionate term was not applied immediately to the ever-growing network, but came almost 30 years after the first tracks were laid and tunnels dug.
It came about because of the combination of tunnels and the price of journeys.
Work was completed on the first part of what would later become the 11th busiest metro system in the world in 1863.
The first line ran underground from what would later become Paddington to Farringdon Street, and was named the Metropolitan Railway.
It was a hit, carrying almost 40,000 passengers on its first day, and the appetite for mass transit would only increase.
In 1868 the District Railway opened, linking the political hub of Westminster to South Kensington.
A trough would be cut out of the earth along the route of the line, and this was then covered to form the distinctive arched tunnels through which even today's trains travel.
This method was used on the two existing lines and used to link them together in a loop through central London, forming the Circle Line.
Wooden, stream-powered locomotives would drag carriages along the newly constructed routes, and those which followed – the City and South London Railway (the first deep-level line in the world) and the Central London Railway.
This latter line, completed in 1900, was the key to the London Underground's universal nickname.
It had a flat fare of two pence for a journey, and the distinctive arched tunnels made an obvious shape.
The line became known alliteratively as the "Twopenney Tube", a name which would eventually cover the whole underground rail network for 117 years – although the two penny fare would, of course, change.