The London Underground is a transit system that serves a large part of Greater London, England and some neighbouring areas. It is the world's oldest underground system, and is one of the longest in terms of route length. Services began on 10 January 1863 on the Metropolitan Railway; most of the initial route is now part of the Hammersmith & City Line. Despite its name, about 55% of the network is above ground. Popular local names include the Underground, more colloquially the Tube, in reference to the cylindrical shape of the system's deep-bore tunnels.
The Underground serves 275 stations and runs over 408 km (253 miles) of line. There are also numerous closed stations. In 2005–2006, 971 million passengers used the Underground and for the first time ever in 2006–2007, over one billion passengers were recorded. As of March 2007, just over 3 million passengers use the Underground each day, with an average of 3.4 million passengers on weekdays.