An underground city is a series of linked subterranean spaces that may provide a defensive refuge; a place for living, working or shopping; a transit system; mausolea; wine or storage cellars; cisterns or drainage channels; or several of these. Underground cities may be currently active modern creations or they may be historic including ancient sites, some of which may be all or partially open to the public.

A metro station or subway station is a train station for a rapid transit system, which as a whole is usually called a "metro" or "subway". A station provides a means for passengers to purchase tickets, board trains, and evacuate the system in the case of an emergency. In the United Kingdom, they are known as underground stations, most commonly used in reference to the London Underground.