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.
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 CentrePoint and internally above ground to Centrepoint, Imperial Arcade, Skygarden, Glasshouse, and the MLC Centre). 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).
Carleton University in Ottawa, Ontario, has a 5-kilometre (3.1 mi) tunnel network which connects ten residence buildings with other buildings on its main campus. The city also has an underground concourse at the Place de Ville office complex in the downtown business district, connecting 4 office buildings containing over 1,000,000 square feet (93,000 m2) of leasable space, and 2 hotels with 900 rooms combined. There are plans to expand the underground network after the Confederation Line, a rapid transit line featuring three downtown subway stations, is completed. It is estimated there will be as many as 20 buildings with direct indoor connections to the downtown subway portion, or 4 million square feet of office space, 1,800,000 square feet (170,000 m2) of retail, 1400 hotel rooms and the Ottawa Convention Centre.
Saskatoon, Saskatchewan – On the campus of the University of Saskatchewan a tunnel system connects several of the buildings on campus, this is augmented with overhead walkways that further extend the network.
Toronto, Ontario ("PATH"), comprises 29 kilometres (18 mi) of walkways and 1,200 shops. It links many important buildings and attractions downtown to six TTC subway stations. PATH accommodates 100,000 pedestrians daily, and PATH businesses host the world's largest underground sidewalk sale once annually.
The PATH network in Toronto is the largest underground shopping complex in the world with 371,600 square metres (4,000,000 sq ft).
Toronto also has a separate, smaller "underground city" connecting several building complexes and two subway stations on Bloor Street.
Winnipeg, Manitoba, has a smaller (mainly commercial office) area located underground in the downtown core below Portage and Main. Several of the downtown office towers have subterranean entrances to the complex allowing employees and visitors to bypass the downtown traffic and avoid the cold winter temperatures Winnipeg regularly experiences. The system links with the skyway system known as the Winnipeg Walkway. Also the University of Manitoba has tunnels for the students to travel from building to building.
Charlottetown, Prince Edward Island, has a tiny system. A pedestrian tunnel connects the Confederation Center of the Arts with the Confederation Court Mall. The mall also connects to The Holman Grand Hotel. The Mall is separated into a main section and a much smaller section connected by an overground walkway. This small section of the mall connects to a 338 spot parking garage, which itself connects to the Homburg Financial Building, and has a separate public entry. Apart from the hotel, two additional office buildings are on top and can be assessed through the mall, a building owned by National Bank, and a building owned by BDC.
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.
Beijing built an extensive tunnel network called the Underground City (Chinese: 地下城; pinyin: Dìxià Chéng) during the Sino-Soviet conflict, supposedly covering 85 km2, falling into disuse in the 1970s. It was opened in 2000 to the public and tourists, but closed in 2008 for renovations. As of 1 July 2009, all "official" remaining entrances appear to be closed.
Guangzhou has at least 16 different underground networks.
Harbin has a number of large, multi-level underground shopping areas, originally built for air defense. The largest is at the roundabout intersection of Xida Zhi street and Hongjun street where three levels of markets following streets from four directions meet under the giant snowflake atrium.
Qingdao has two small underground shopping areas, one at the head of the Zhanqiao (pier) and one west of the Qingdao guest house.
Shanghai has a few underground networks, most notably at People's Square metro station, wherein the line 2 station has a second mezzanine full of shops and Line 1 is connected to a large underground shopping gallery at its south end. Shanghai Science Museum stop on line 2 has a large underground shopping area, known for its imitation goods. Huangpi Road South and Xujiahui stations are directly connected to shopping centers, and the Lujiazui station is connected to the Bank of China tower.
Shenzhen has quite a few underground shopping malls:
In Helsinki centre, underground shopping areas cover the central railway station area including the underground shopping centre Asematunneli, and two subway stations, Rautatientori and Kamppi. The Kamppi metro station is integrated with the Kamppi Center (55,000 m2 (592,015 sq ft)) long-distance bus terminal, freight depot and internal parking area, all underground. It features a six-storey shopping complex and a central bus terminal for local buses. Together with these, the Forum shopping centre, Sokos and Stockmann department stores are all connected together with underground walkways featuring shops and other services.
The mines of Paris are several disconnected networks of more than 300 km (186 mi) of mining tunnels started in the 13th century and dominated by the "large south network" on the Left Bank, of which 1.7 km (1 mi) was repurposed from 1786 as the Catacombs of Paris ossuary, the final resting place of 6-7 million Parisians.. Other parts of the tunnel network have long been closed off for safety reasons, but the tunnels have served as safe passageways during war and revolution, routes for urban explorers, and venues for unauthorized cultural activities.
La Défense, the major business district northwest of the city center built in the 1960s, has an extensive network of commercial passageways under a vast plaza centered on the Gare de La Défense and radiating out to connect with surrounding buildings, notably the large Les Quatre Temps shopping center and CNIT, but enabling access to many buildings of the district.
The Forum des Halles is a partially underground multilevel commercial and shopping center connected to the massive underground transit hub Chatelet-Les-Halles. Opened in 1979, passageways extend west under the Jardin Nelson Mandela for several city blocks, and within the fare zone of the Paris Metro, stretch half a kilometer south to the banks of the River Seine.
Naours: 33 metres (108 ft) below the Picardy village of Naours (between Doullens and Amiens), the 28 galleries (2,000 metres (6,562 ft) long) of an ancient limestone quarry (exploited since the 3rd century c.e.) have long been used as shelter by the population seeking refuge from invaders. Occupied by the Triple Entente forces during WWI, and then used as headquarters by the German Army during the WWII occupation of France, the galleries are now open to visitors.
Arras: During World War I, the French and British Army built a tunnel system (boves) based upon the already existing adits from a formerly used quarry. Scots and Englishmen used the quarry below the quarter Saint-Sauveur and called it "Glasgow", "Manchester" and "Liverpool". The New Zealand Army (bonded with the British Army) used the quarry for constructing another tunnel system below the quarter Ronville and called it "Wellington", "Auckland" and "Nelson". Soldiers from these armed forces hid in the tunnels for several days. On April 9, 1917, 5:30 am, 24,000 soldiers sortied from there to encounter German troops.
Berlin: Several buildings on the east side of Friedrichstraße, from Quartier 205 (Friedrichstraße 70) northwards up to Galerie Lafayette are connected to each other. Businesses are on both sides of the underground street so that it appears to be inside a building all the time, even when it crosses Taubenstraße underground.
Extensive underground bunkers still exist all throughout the city centre, although they are largely sealed off and closed to the public.
All of the buildings at the large Charité Virchow campus are connected via a system of spacious tunnels. In addition to employee use, particularly during inclement weather, there is also a rudimentary robotic system via which mail and packages are delivered across campus.
Frankfurt am Main: Underground shopping malls, called 'B-Ebene' in Frankfurt, of considerable size are found at Hauptbahnhof (central station) and Hauptwache.
Hamburg: The Jungfernstieg and Rathaus subway stations and several entrances are connected by pedestrian tunnels, some of them contain shops. There is a direct access to the Europapassage shopping mall.
Munich: Underground shopping mall at Karlsplatz/Stachus. This combines the underpass that leads to the pedestrian area and the entry to the subway system.
Stuttgart: There is a large underground shopping mall (Klettpassage) connecting the Hauptbahnhof (central station) with the main shopping street, the Königstraße.
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.
Historical underground cities of Persia include Samen, Nushabad, and Kariz.
Kish: An underground city by the name of Hidden Pearl was constructed of roads interlinked 20 meters (66 ft.) under the ground. Shops and restaurants are planned to be built.
Tehran: Tehran has made a series of underground pathways in and around Vali-e Asr Metro Station and is in planning stage to increase commercial activity in newly built buildings in the central part of the city. The priorities for future development are expansion of underground connections around Haft-e Tir Metro Station and Meydan-e Vali-e Asr Metro Station.
Isfahan: With the completion of Imam Hosein Metro Station, and Jahan Nama Complex, and their eventual underground connection, there would be an underground complex of a length of 300 m formed in Isfahan downtown area. Also, not underground per se, with Imam Ali Square's street network being dug underground and a large open space plaza being constructed on the top, the plaza is connected through a series of covered bazaar pathways of a length exceeding 4 km, connecting it to Naqsh-e Jahan Square.
Nushabad: Nushabad has an underground city that served as a refuge during wars.
Osaka has enormous underground networks in the Umeda, Namba, and Shinsaibashi districts, in which Umeda alone includes over 1,200 retail stores and restaurants, as well as subway and intercity rail stations.
Tokyo has numerous networks of connecting passages surrounding subway stations that span a few blocks for commuters. Stations such as Shinjuku and Shibuya have underground shopping malls. Shinjuku in particular has a reputation for being so large and complex that even local Japanese get lost there. The top five largest underground "cities" (地下街, chikagai) in Japan are all shopping districts:
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.
Maastricht: Originally a casemate, the kazematten of Maastricht form a 14 km (9 mi) long network of tunnels underneath the western part of the city. This tunnel network has mainly been used for military purposes. The main construction period of these tunnels lasted from 1575 to 1825. The newest sections of the tunnel network were dug as late as the middle 20th century, built in the Cold War as a shelter for citizens in the event of a nuclear strike on the city.
Caves of Maastricht are a far more extensive, system of tunnels with a length over 200 km (124 mi) and 20,000 individual corridors lies just west of Maastricht; (Dutch: Grotten van Maastricht). These man-made 'caves' were used as Marl quarries from the 13th century onwards. In World War II, these caves were used to hide large quantities of paintings from the Germans, even including the Nachtwacht. In 1944, construction started on a large public shelter that could have housed 45,000 persons in these caves. The project never saw its completion due to the liberation of Maastricht in the fall of the same year.
In Barcelona, there is an abandoned underground mall near Plaça de Catalunya called Avinguda de la Llum, closed since 1990, which had originally been part of a more ambitious project to build an underground city under the centre of Barcelona. Also, some Metro stations or connecting lines in the same station are connected by underground passages over a block in length.
Legend has that the many caves and tunnels in Subterranean Toledo under the old part of Toledo were connected and were used by occultists.
By walking through Stockholm subway stations and indoor shopping malls it is possible to walk indoor through the central business district, partly underground, from Arsenalsgatan subway entrance (Kungsträdgården station) to Kungsbron (north entrance to Cityterminalen bus terminal), covering a distance of between 1 and 2 kilometres.
Geneva contains a large underground shopping centre which also acts to connect separate sections of surface shops.
Cappadocia contains several historical underground cities carved out of unusual geological formations formed via the eruptions of ancient volcanoes. The cities were initially inhabited by the Hittites, then later by early Christians as hiding places. They are now archeological and tourist sites but are not generally occupied (see Kaymakli Underground City, Derinkuyu underground city, Özkonak Underground City, Mazı Underground City). The latest large underground city was discovered in 2007 in Gaziemir, Güzelyurt. It was a stopover on the Silk Road, allowing travelers and their camels to rest in safety underground, in a 'fortress' equivalent to a modern hotel.
A typical view from inside the underground city in Derinkuyu, one of the largest underground complexes in Cappadocia. There are few artifacts left from the original builders, mainly just large rocks that were used to block the passage for intruders. Most of the "cities" are corridors, but some places there are rooms large enough to live in, and some rooms that have the cross-like shape of a church, which is probably exactly what they were.
Istanbul boasts the Roman cisterns, built 2,000 years ago for water storage it is now a tourist attraction.
Kyiv: An underground concourse extends underneath Khreschatyk Street from Maidan Nezalezhnosti to Ploscha L'va Tolstoho. The concourse connects to the Kyiv Metro and to the Globe shopping mall beneath Maidan Nezalezhnosti.
Odessa: A ramified tunnel network made from the former quarries that is famous as Odessa Catacombs covers the historical center of Odessa and some suburban areas.
Corsham, Wiltshire, is the location of the Central Government War Headquarters, code name 'Burlington'. Built in the late 1950s in response to the increasing threat of nuclear warfare during the Cold War, the 35 acres (14 ha) subterranean site was designed to be the main emergency government war headquarters of the UK outside London and safely house up to 4,000 central government personnel in the event of a nuclear strike.
Dover contains a series of interconnecting tunnels, that honeycomb both sides of the Dover Valley, carved into the chalk cliffs. These date from Ancient times at Dover Castle through to Napoleonic, Second and Cold War installations. The Dover Castle complex is the larger, going at least six levels deep and includes a hospital, troops quarters, offices and storage and channel view points. The southern tunnels are mixed between Napoleonic War-era defences (see Dover Western Heights) and Second World War-era defences, with some seafront air raid shelters still used for shop storage today. Many have fallen into disrepair and are now closed to the public, but many are still open.
There are extensive underground constructions across Britain, such as Chislehurst Caves, built or repurposed as air-raid shelters during World War II.
Edinburgh's old town has extensive rooms, tunnels and chambers beneath some areas; of particular note are the Edinburgh Vaults, where overcrowding led people to construct elaborately interconnected buildings in the vaults of the city's South Bridge.
In Southport, Merseyside, Nevill Street has the remainders of an underground shopping street, which can now only be accessed from the cellars of buildings on the current street, which was raised by one storey from the original level. One end of the underground street ended at the Marine Lake, close to the pier entrance.
In Liverpool, the Williamson's tunnels included the site of an 'underground house' complete with windows (concealed by work for public opening) and an extant and partially excavated 'banqueting hall'.
Atlanta, Georgia: The "Underground" represents the original surface level of downtown Atlanta; the present streets are raised roadways (viaducts) built in the 1920s. The shopping center Underground Atlanta, taking advantage of the former street-level storefronts, covers six city blocks and includes retail shopping and restaurants. It was begun in 1968 and re-opened in 1989 after a financially forced closure.
Boise, Idaho: The downtown sector's Capitol Mall Complex consists of a large system of networked tunnels that connect all the state buildings. The tunnels have walkways and vehicle passageways. The underground area boasts a geothermal power plant, a banking system, extensive dining areas, parking, a dedicated mail room for the Capitol Mall Complex and a fallout shelter. The main hall is decorated with art from local artists that was collected over a thirty-year time frame. This collection is not often seen by the public as access is limited. Recent remodeling of the Idaho State Capitol Building has added new underground wings that are linked to the Capitol Mall. These new wings have offices, meeting rooms, and records storage areas and were designed to support the Idaho legislature when it is in session.
Dayton, Ohio: The academic section of Wright State University's campus contains nearly two miles (3.18 kilometers) of tunnels  that link 20 of the 22 buildings. The tunnels were designed to connect the campus without imprinting the surrounding land.
Duluth, Minnesota: The city's downtown sector has an extensive network of skyways and tunnels connecting its buildings, including the Federal Courthouse and Convention Center (DECC).
Havre, Montana: The city has an underground area, called "Havre Beneath the Streets", that operates as a tourist attraction.
Houston, Texas: The seven-mile (11 km) Houston tunnel system is set about twenty feet below Houston's downtown street system and is composed of underground passageways which, with above-ground skywalks, link office towers to hotels, banks, corporate and government offices, restaurants, retail stores, and the Houston Theater District. Only one building, Wells Fargo Plaza, offers direct access from the street to the Tunnel; otherwise, the Tunnel can only be entered via street-level stairs, escalators, or elevators inside a building connected to it.
Irvine, California: A network of large tunnels running beneath the University of California, Irvine connects many of the campus' major buildings to a central utility plant. These tunnels are only accessible to maintenance staff, although there are also publicly accessible tunnels which intersect the utility tunnels, such as the one that goes between the main Information & Computer Science building and the Engineering Tower.
Kansas City, Missouri: The city's SubTropolis is a 55,000,000 square foot (5,060,000 m³), 1,100-acre (4.5 km²) underground business complex running along the Missouri River. The space was originally part of the Bethany Falls limestone mine, and was later repurposed for use as a commercial underground storage facility.
Oklahoma City, Oklahoma: The Oklahoma City Underground (formerly the Oklahoma City Conncourse) is a tunnel system connecting nearly all of the downtown buildings in a 20-square-block area. It is one of the most extensive all-enclosed pedestrian systems in the U.S., extending three-quarters of a mile and connecting over 30 downtown buildings via tunnels or skyways. The original tunnel link was built in 1931 and the system was extended in the 1970s. It underwent a $2 million renovation in 2006–2007, after which the Conncourse was renamed the Underground.
Richmond, Virginia: There are a series of connected tunnels between state government buildings in the city of Richmond. Certain passageways are locked off but a good portion of the tunnels are accessible from buildings. The purpose of the tunnels is not generally known; the two most common explanations are that they were built to allow people to move between buildings in inclement weather or that they were built as part of an emergency evacuation plan.
Rochester, Minnesota: The Mayo Clinic's buildings in the downtown area are interconnected with tunnels and skyways. Other businesses are along the corridors, including a number of hotels that often house clinic patients. It is often called a subway, although there are no underground rails in the city.
Vicksburg, Mississippi: During the Siege of Vicksburg in 1863, Union gunboats lobbed over 22,000 shells into the town, destroying nearly all of the town's housing. As a result, over 500 caves were dug into the clay hills surrounding Vicksburg. The Union soldiers later gave the town the nickname of "Prairie Dog Village" due to these caves.
Walt Disney World, Florida (southwest of Orlando, Florida) has a network of utility tunnels used by its employees for transportation between venues, rest areas, staff preparation, and first aid. The main system is under the Magic Kingdom theme park. Other tunnels lie underneath Future World at Epcot.