Underground city

​An underground city is a series of linked subterranean spaces that may provide a defensive refuge, a place for living, working and shopping, a transit system, cisterns, wine or storage cellars, drainage channels, or several of these. The term may also refer to their tunnels that connects buildings below street level which may house offices, 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.

Hong Kong
Many MTR stations in Hong Kong form extended underground networks connecting to buildings and at the basement of major shopping malls in the area above. Notable examples are Central-Hong Kong, and Tsim Sha Tsui-East Tsim Sha Tsui stations. Additional underground networks are planned in Causeway Bay in 2006. Studies are ongoing for Tsim Sha Tsui, Kowloon Park, Victoria Park, Causeway Bay, Happy Valley, Admiralty, Wan Chai and Hong Kong Park.

Singapore

 * Extensive underground networks exist around most major stations in the MRT, such as the one at Raffles Place MRT Station with direct underground connections to 19 buildings in the busy Raffles Place area. Expanding the network of the first phase linking the station towards One Raffles Quay and Marina Bay Financial Centre for a complex underground pedestrian network across the entire downtown area.
 * The CityLink Mall offers over 60,000 sq ft of the retail space and connects City Hall MRT Station together with Esplanade MRT Station, Marina Square, Suntec Convention Centre and Esplanade.
 * The Orchard Road shopping belt is connected by underground linkways often with commercial space, particularly along Orchard, Somerset and Dhoby Ghaut stations. Plans are also in place to link up Bras Basah Road first, before going to Suntec City, Chinatown and Bugis area.
 * There are also several tunnels in Tampines (Ashford Walkway Tunnel), Tampines West (Tampines West Tunnel) and Bukit Panjang.

Taiwan
Taiwan has underground streets connecting two or more metro stations. The notable one example is Taipei City Mall.

United Kingdom

 * In Canary Wharf station, adjacent office towers and shopping malls are connected underground. It is possible to access two stations on the Docklands Light Railway without going inside.
 * There are extensive rooms, tunnels and chambers which is called Churchill War Chambers.