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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.
The term may also refer to a network of tunnels that connects buildings beneath street level that may house office blocks, shopping malls, metro stations, theatres, and other attractions. These passages can usually be accessed through the public space of any of the buildings connecting to them, and sometimes have separate entries as well. This latter definition encompasses many modern structures, whereas the former more generally covers tunnel systems from ancient times to the present day.
Underground cities are especially functional in cities with very cold or hot climates, because they permit activities to be comfortably accessible year round without regard to the weather. Underground cities are similar in nature to skyway systems and may include some buildings linked by skyways or above-ground corridors rather than underground.
Some cities also have tunnels that have been abandoned.   
Buenos Aires, capital of Argentina, has an extensive number of underground cities in its Subte. Most stations have small shops, bars and kiosks, while main hubs interconnect through underground pedestrian walkways with railroad stations, governmental buildings, or shopping centres. Some have additional mall-like mezzanine levels, with the Centro Obelisk of Buenos Aires area (three lines, four underground levels), Estación Retiro, Estación Constitución, Estación Once, and Federico Lacroze railway station being the most important ones.
Sydney has a series of underground shopping malls around the Town Hall underground station. The tunnels run south to the George Street cinema district, west under the town hall, and north to Pitt Street Mall through the Queen Victoria Building. The northern branch links Queen Victoria Building with Galleries Victoria, Sydney Central Plaza (which in turn links underground to Westfield Sydney and internally above ground to Centrepoint, Imperial Arcade, Skygarden, Glasshouse and 25 Martin Place). The linked centres run for over 3 km (2 mi). In 2005, Westfield Corporation submitted a development application to link Sydney Central Plaza underground with 3 other properties on Pitt Street Mall and extend the tunnel network by a further 500 m (1,640 ft) or more.
Perth has a small network of underground shopping malls running from the St Georges Terrace entrance to Trinity Arcade to Murray Street Mall.
The small town of Coober Pedy in northern South Australia has numerous underground residences and other facilities. The area was and is extensively mined for opal, and the settlers lived underground to escape the scorching daytime heat, often exceeding 40 °C (104 °F).
The cold-winter northern continental climate of much of Canada makes underground pedestrian malls particularly useful in many cities.
Santiago has some elements of an underground city in its "Metro" subway system. While all stations have a small mezzanine level above the tracks for ticket purchase, some key stations have extensive areas of shops and kiosks in addition. Some stations even have an additional mall-like level between the street and the mezzanine levels.
In Thessaloniki, Hellenistic, Roman, Byzantine, and Ottoman monuments coexist underground and several of these have been discovered and emerged. Monuments such as the Catacombs of St. John and the underground museum of the Agora have limited access and many are not permitted to enter even to the locals by the Archaeological Department of the Thessaloniki underground Metro project. 
Many MTR stations in Hong Kong form extended underground networks connecting to buildings and at the basement of some major shopping malls in the area above. The stations themselves house a number of retail shops. Notable examples are the Central–Hong Kong stations and the Tsim Sha Tsui–East Tsim Sha Tsui stations. Only rarely are there not any shops. Additional underground networks have been proposed for Causeway Bay in 2006 and in Kwun Tong under Hoi Yuen Road in 2010. As of 2014 [update] , studies are underway for underground networks in Tsim Sha Tsui, Kowloon Park, Victoria Park, Causeway Bay, Happy Valley, Admiralty, Wan Chai and Hong Kong Park. In 2017, The Development Bureau announced that two underground streets will be constructed, which would connect San Po Kong, Kai Tak station and Sung Wong Toi station.
Historical underground cities of Persia include Samen, Nushabad, and Kariz.
Jordan 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 from the subway to the train station without going outside.
Taipei has underground streets connecting two or more metro stations. In addition, there is a large underground shopping mall near the main train station.
Other parent categories from the same field:
Types of underground spaces and people, and related topics:
A people mover or automated people mover (APM) is a type of small scale automated guideway transit system. The term is generally used only to describe systems serving relatively small areas such as airports, downtown districts or theme parks.
A skyway, skybridge, skywalk, or sky walkway is an elevated type of pedway connecting two or more buildings in an urban area, or connecting elevated points within mountainous recreational zones. Urban skyways very often take the form of enclosed or covered footbridges that protect pedestrians from the weather. Open-top modern skyways in mountains now often have glass bottoms. Sometimes enclosed urban skywalks are made almost totally from glass, including ceilings, walls and floors. Also, some urban skyways function strictly as linear parks designed for walking.
In American English, walkway is a composite or umbrella term for all engineered surfaces or structures which support the use of trails.
Nicollet Mall is a twelve-block portion of Nicollet Avenue running through downtown Minneapolis, Minnesota, United States. It is the shopping and dining district of the city, and also a pedestrian mall and transit mall. Along with Hennepin Avenue to the west, Nicollet Mall forms the cultural and commercial center of Minneapolis.
Downtown Pittsburgh, colloquially referred to as the Golden Triangle, and officially the Central Business District, is the urban downtown center of Pittsburgh. It is located at the confluence of the Allegheny River and the Monongahela River whose joining forms the Ohio River. The triangle is bounded by the two rivers.
RÉSO, commonly referred to as the Underground City, is the name applied to a series of interconnected office towers, hotels, shopping centres, residential and commercial complexes, convention halls, universities and performing arts venues that form the heart of Montreal's central business district, colloquially referred to as Downtown Montreal. The name refers to the underground connections between the buildings that compose the network, in addition to the network's complete integration with the city's entirely underground rapid transit system, the Montreal Metro. Moreover, the first iteration of the Underground City was developed out of the open pit at the southern entrance to the Mount Royal Tunnel, where Place Ville Marie and Central Station stand today.
Path is a network of underground pedestrian tunnels, elevated walkways, and at-grade walkways connecting the office towers of Downtown Toronto, Ontario, Canada. It connects more than 70 buildings via 30 kilometres (19 mi) of tunnels, walkways, and shopping areas. According to Guinness World Records, Path is the largest underground shopping complex in the world, with 371,600 square metres (4,000,000 sq ft) of retail space which includes over 1,200 retail fronts (2016). As of 2016, over 200,000 residents and workers use the Path system daily with the number of private dwellings within walking distance at 30,115.
Downtown Brooklyn is the third largest central business district in New York City, and is located in the northwestern section of the borough of Brooklyn. The neighborhood is known for its office and residential buildings, such as the Williamsburgh Savings Bank Tower and the MetroTech Center office complex.
The Chicago Pedway is a network of tunnels, ground-level concourses and bridges connecting skyscrapers, retail stores, hotels, and train stations throughout the central business district of Chicago, Illinois.
The Houston tunnel system is a network of subterranean, climate-controlled, pedestrian walkways that links 95 full city blocks 20 feet (6 m) below Houston's downtown streets. It is approximately six miles (9.7 km) long. There are similar systems in Chicago, Dallas, Oklahoma City, Montreal, and Toronto. Architectural historian Stephen Fox has stated that the idea for the tunnel system came when the Bank of the Southwest Building was "linked by tunnel to the 1010 Garage and the Mellie Esperson Building" in 1961.
Kuala Lumpur City Centre (KLCC) is a multipurpose development area in Kuala Lumpur, Malaysia. KLCC refers to the area within and surrounding the KLCC Park but the term has also been widely used by buildings nearby to the vicinity.
Downtown Toronto is the main central business district of Toronto, Ontario, Canada. Located entirely within the district of Old Toronto, it is approximately 16.6 square kilometres in area, bounded by Bloor Street to the northeast and Dupont Street to the northwest, Lake Ontario to the south, the Don Valley to the east, and Bathurst Street to the west. It is also the home of the municipal government of Toronto and the Government of Ontario.
The Winnipeg Walkway System, also known as the Winnipeg Skywalk, is a network of pedestrian skyways and tunnels connecting a significant portion of downtown Winnipeg, Manitoba.
Downtown Edmonton is the central business district of Edmonton, Alberta. Located at the geographical centre of the city, the downtown area is bounded by 109 Street to the west, 105 Avenue to the north, 97 Street to the east, 97 Avenue and Rossdale Road to the south, and the North Saskatchewan River to the southeast.
The Edmonton Pedway system is a network connecting office buildings, shopping centres, and parkades in downtown Edmonton, Alberta, Canada. It consists of approximately 13 kilometres (8.1 mi) of year-round climate-controlled tunnels, and walkways between the second floors of buildings, approximately 15 feet (4.6 m) above ground. The main network connects more than 40 buildings and parkades, and three of the five Edmonton Light Rail Transit (LRT) stations in the downtown area.
The Minneapolis Skyway System is an interlinked collection of enclosed pedestrian footbridges that connect various buildings in 80 full city blocks over 9.5 miles (15.3 km) of Downtown Minneapolis, enabling people to walk in climate-controlled comfort year-round. The skyways are owned by individual buildings in Minneapolis, and as such they do not have uniform opening and closing times. The 9.5 miles of skyway are comparable to the Houston tunnel system, the systems in Canadian cities such as Toronto's PATH, Montreal's Underground City, Calgary's 11-mile +15 system and the 8-mile Edmonton Pedway system.
Westlake station is a light rail station that is part of the Downtown Seattle Transit Tunnel in Seattle, Washington, United States. The station is located under Pine Street between 3rd and 6th avenues in Downtown Seattle, near Westlake Center and Westlake Park. It is served by the 1 Line, part of Sound Transit's Link light rail system, and also connected above ground by buses at several stops, the South Lake Union Streetcar, and the Seattle Center Monorail.
The Dallas Pedestrian Network or Dallas Pedway is a system of grade-separated walkways covering thirty-six city blocks of Downtown Dallas, Texas, United States. The system connects buildings, garages and parks through tunnels and above-ground skybridges. The network contains an underground city of shops, restaurants and offices during weekday business hours.
The Line C is a line on the Frankfurt U-Bahn. It consists of the U6 and the U7.
Lakeview Square is a full-block, mixed-use development in downtown Winnipeg, Manitoba. It opened in 1974 and was developed by Lakeview Properties, Ltd.