The early tunnels were made by a cut-and-cover method but this caused a lot of destruction above and disruption. The first trains were steam powered requiring ventilation to the surface using shafts. In later years and advances in technology the tunnels were built deeper underground.
During World War I many of the stations were used as air raid shelters. (My mother remembers sitting in one over at Earls Court!)
The custom built escalators in the stations are some of the longest in Europe. Angel Station has the longest at 197 feet long and a 90 feet rise.
Today the underground serves 270 stations by rail, Fourteen of which are outside Greater London with approximately 2.7 million journeys made every day. Victoria is the busiest station.
The phrase "Mind the Gap" dates to 1968 and in most stations is the original voice recorded. The 'roundel' logo was adopted in 1908 to make the stations easily recognizable. The nickname the tube was born when it was referred to as the two-penny tube due to cost and shape of the tunnels.
An estimated half a million mice live in the underground!