The London Underground (also known by its nickname “theTube”) is a public rapid transit system serving Greater London and some adjacent parts of some counties in the United Kingdom.
The world’s first underground passenger railway system opened in London this date in 1863. The network has expanded to 11 lines and in 2015-2016 carried 1.34 billion passengers, making it the world’s 11th busiest metro system.
The system’s first tunnels were built just below the surface, using the cut and cover method. Later, smaller, roughly circular tunnels (which gave rise to its nickname “the Tube”) were dug through at a deeper level. The system has 270 stations and 250 miles of track. Despite its name, only 45% of the system is actually underground in tunnels, with much of the network in the outer environs of London being on the surface. In addition, the Underground does not cover most southern parts of Greater London, with less than 10% of the stations located south of the River Themes. (from Wikipedia and cute-calendar.com)