The Underground is a term commonly used to refer to the London metro system, but there are a number of other synonyms for this iconic feature of the city. Some popular alternatives include the Tube, the subway, the metro, and the railway. Other phrases that might be used to describe the same thing include the subterranean transport network, the metropolitan transit system, or simply the transport network. While each of these terms has its own unique connotations and associations, they all help to capture the essential character of this vital means of transportation that has helped to shape the modern city of London.