The London Underground is a rapid transit(交通运输系统) system in the United Kingdom, serving a large part of Greater London. The underground system is also known as the Tube, due to the characteristic shape of the subway tunnels. It all started in the mid-1800s. The Tube was the world’s first underground train system, with the first section opening in 1863. Since then it has grown to an underground masterpiece(杰作) of 12 lines, 275 stations, and over 250 miles of rail track, 45% of which is underground. It is the fourth largest metro system in the world in terms of route miles. It also has one of the largest numbers of stations. As an affordable and easy way to get around London, the Tube remains the first choice for millions of commuters each day, as well as tourists visiting the city on holidays. The Tube has been an international icon for London. The London Underground celebrated its 150 years of operation in 2013, with various events marking the milestone(里程碑).