The first underground trains were pulled by steam engines, which sounds strange because steam engines make a lot of smoke. Passengers got covered in soot. There were vents in the tunnels to let the smoke out, but the carriages were still quite gloomy. Modern Tube trains run on electricity, which is much cleaner.
The Tube has 11 lines and 272 stations. Each line has its own colour on the map: the Central Line is red, the Piccadilly Line is dark blue, the District Line is green. The famous Tube map is so cleverly designed that people study it as a brilliant example of how to show information.
Some of the stations are very deep underground - Hampstead station is 58 metres down, deeper than 14 double-decker buses stacked on top of each other. During tough times in the past, people used the deep platforms as safe places to sleep.
The Tube has its own little customs. 'Mind the gap' is the famous warning between the train and the platform. People stand on the right side of the escalator so people in a hurry can walk past on the left. And the round red, white and blue 'roundel' logo is one of the most recognised symbols in the world.