Mind the gap -- it's the 150th anniversary of the London Underground, and Google is celebrating with a totally tubular doodle.
For today only, the Google logo on the search engine's home page reworks Harry Beck's truly iconic map of the London Underground, designed in 1931 and cleverly doing away with geographical accuracy to more clearly show the layout of the network.
Knocked up in his spare time, Beck's instantly recognisable, hugely influential and widely copied topological map earned him just five guineas at the time. It has since been voted one of the best British designs of the 20th century, second only to Concorde.
The Tube began with a steam locomotive on the Metropolitan Line on this day in 1863, the world's first underground railway. Originally built using the cut-and-cover method -- digging a trench at ground level, then covering it over to form a tunnel -- the Underground as we know it became possible with the development of tunnelling shields, allowing deeper tunnels to be dug with no disruption at street level.
In 1890, the City and South London Railway -- now part of the Northern Line -- became the world's first deep-level electric railway.
In 1900, the Central Line was nicknamed the "Twopenny Tube" for its fare and round tunnels, and the moniker stuck. Today the Tube serves 270 stations on 11 lines -- only 27 stations south of the river, if you please -- making 1.71 billion passenger journeys a year. As of last year, 70 stations offer free Wi-Fi too.
Ironically, more of the London Underground network is above ground than below.
The Google doodle is a playful remix of the Google logo found on the search giant's otherwise crisp white home page, which pops up to mark a special day, or the birthday of famous folk. Click play below to see our favourites, including a shooting star leaping through the skies like a tiger, defying the laws of gravity.
Who would you like to see celebrated by Google? Doodle in the comments, or on our Facebook page.