Underground city

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 entirely 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 centres, 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.
Many MTR stations in Hong Kong form extended underground networks connecting to adjacent buildings and at the basement of some major shopping malls in the area above. In addition, the stations themselves often house a number of retail shops.
Among such, the largest underground network consists of numerous out-of-system passageways within the station complex connecting the Tsim Sha Tsui–East Tsim Sha Tsui stations, government-owned pedestrian underpasses and basements of shopping malls, including the K11 Art Mall. In whole, the underground network spans almost the entirety of the retail heart of Tsim Sha Tsui, from Canton Road to the West and Chatham Road South to the East, and from Victoria Harbour to the South and Cameron Road to the North.
Previously, additional underground networks have been proposed for Causeway Bay in 2006 and in Kwun Tong under Hoi Yuen Road in 2010. As of 2014, 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. However, as of 2024, only the network in Tsim Sha Tsui had been built.
In 2017, The Development Bureau announced that two underground streets will be constructed in Kowloon City District, which would connect Kowloon City, San Po Kong, Kai Tak station and Sung Wong Toi station. The system would be known as “Kai Tak Underground Shopping Street.” As of 2024, the only portion constructed and operational is the basement portion of the AIRSIDE mall, which connects to Kai Tak Station.
Historical underground cities of Persia include Samen, Nushabad, and Kariz.
Taipei has underground streets connecting two or more metro stations. In addition, there is a large underground shopping mall near the main train station.
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 (1.9 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).
Melbourne is said to have the largest underground tunnel system in the world spanning over 1,500 km (930 mi), with the clandestine group known as the Cave Clan who meet regularly to explore, vandalise, socialise and map out this network. The vast majority of these tunnels are decommissioned or for stormwater purposes.
In Thessaloniki, Hellenistic, Roman, Byzantine, and Ottoman monuments coexist underground and several of these have been discovered and merged together. 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.
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.
The cold-winter northern continental climate of much of Canada makes underground pedestrian malls particularly useful in many cities.
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.
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.