Google is showing a doodle on its UK homepage today, celebrating the 150th anniversary of the London Underground. For those unfamiliar with the Underground, it’s a transit system in the UK, which serves a big part of Greater London, as well as some parts of Buckinghamshire, Hertfordshire and Essex. It’s often referred to as the Tube.

There are 11 lines, and 270 stations. The Underground sees about 3.23 million daily riders, with 3.66 million on weekdays, based on data from LU Performance Data Almanac.

If you’re interested in learning more about the Tube, here’s 45-minute National Geographic documentary about it:

Google has a Knowledge Graph result for London Underground, but it only displays image results, and a brief description from Wikipedia, along with the obligatory “People also search for” section. By the way, people who search for London Underground, apparently also search for: Paddington station, London Heathrow Airport, Central London, Liverpool Street station, and St Pancras railway station.