Airports of London cannot be expanded due to the docks on either side. It is also the only airport serving London which does not operate at night. A light rail service from London City Airport DLR station, which adjoins the terminal building of London City Airport, links it to the financial district of the City of London at Bank and Monument stations, which offer interchanges with London Underground. Located in the London Borough of Hillingdon, Heathrow is by far the largest of London's airports and considered the main gateway into the United Kingdom for non-European visitors. Heathrow has four terminals and two parallel
Underground city boasts the city of Petra — "rose-red city, half as old as time" — carved from the living rock. Guanajuato city was built over old silver mines, some of which are used as roads. The Mexico DF metro system has many underground pedestrian walkways connecting stations. In general, many large railway stations house underground hallways featuring shops, restaurants, banks and money exchange offices. A striking example of such stations would be the main hallway of the Amsterdam central station, which connects to the city's metro system, although due to renovation and re-building it was temporarily (2012–2015) not possible to walk