Underground works

Many large cities have transit systems where the stations and tracks are below the surface, whose public areas can be accessed for the price of a standard fare. Some systems of particular note are London, New York, Paris, Berlin, Moscow, and the worlds longest art Gallery in Stockholm, but other systems are also of interest to transit fans.

Whilst some Urban Rail systems occasionally run limited enthusiast tours 'behind the scenes', the health and safety considerations of a working railway, the current 'security' climate in many regions, coupled with the confined operational environment, mean that these are rare, and typically take place outside operational periods. Specialist Enthusiast publications may advertise these type of tours, but booking early is essential owing to limited numbers.

See also: Fortification, Military tourism

Military facilities – even if seemingly abandoned or out of use – often remain highly sensitive sites. An unexpected or unannounced visit could at best lead to a lengthy interrogation, with considerably worse outcomes depending ultimately on the mood of the personnel you encounter. You should make formal contact in writing with the relevant military authorities as soon as you have firm travel plans. Do not be disappointed if a planned or agreed visit has to be cancelled or curtailed for operational and security reasons, or if you are denied access without any reason at all being provided.

• 🌍 Coober Pedy. An opal mining town in the South Australian Outback that is almost fully underground, as this is the only way to ensure human habitation in the harsh climate. (updated Dec 2015)

• Fremantle Prison Tunnels. A tunnel system in Western Australia underneath the Fremantle Prison (updated Dec 2015)

• 🌍 Sarajevo War Tunnel Museum, Sarajevo. A house converted to a museum sitting at the entrance of the tunnel that linked the city to the airport, thus providing the only lifeline for the inhabitants of the city during the Siege of Sarajevo which lasted for three years in the context of the Yugoslav Wars of the 1990s. (updated Dec 2015)

• 🌍 Diefenbunker - Canada's Cold War Museum, Carp, Ontario, toll-free: +1-800-409-1965. Self-guided tours 11AM-4PM daily, guided tours by reservation only. Built to protect the government from nuclear attack, this once-secret bunker is now a museum and National Historic Site of Canada. See the Ottawa and nuclear tourism articles for details $14 adults, $13 seniors, $10 students , $8 youth 6-18, $40 families (2 adults plus 3 youth), free for children 5 and under. 

• 🌍 Musée des Égouts de Paris (Entrance opposite 93, quai d'Orsay near the Pont d'Alma, Métro: Alma-Marceau), ☏ +33 1 53 68 27 81. For an interesting take on Paris, check out its underground sewer system. See swords found in the sewers over the years and get an appreciation for what it takes to keep Paris running. Full fare: €4.30, Student: €3.50. 
• The Mines of Paris often erroneously termed the Catacombs.

During the second world war, constant allied bombardment which had destroyed several important military factories forced the Nazis to dig deep and hide their military infrastructure underground. Nazi architect and minister of armament Albert Speer was the leading force behind those efforts often brutally abusing and outright murdering the forced laborers used for those projects. While many of those structures were deliberately destroyed after the war, a few survive to this day and can be visited.

• 🌍 Dora Mittelbau concentration camp, Near Nordhausen. Site of the building of the V2/Aggregat4 rocket which was fired on London and later other cities in a futile effort to turn the tide of the lost war. Conditions for the forced laborers were horrific and more people died building the rockets than in their use. (updated Apr 2018)
• 🌍 Berliner Unterwelten. The "Berlin Underworlds" consists of various structures built below Berlin throughout its troubled history. The Verein of the same name offers a large amount of various tours. (updated Jan 2019)

• The First World War on the Alpine Front pitted Austria-Hungary against Italy and involved a lot of mines and tunnels, some of which are still visible a century later.
• Napoli Sotterrano (Napoli Underground), Piazza san Gaetano (Via Tribunali at San Paolo Maggiore). Caves under the city center created by mining from the first Greek settlers 2000 years ago. (updated Jun 2016)

• Metropolitan Area Outer Underground Discharge Channel in Kasukabe, Saitama prefecture is a monumental underground flood-control system finished in 2009. It was constructed to prevent damages caused by the perennial floods which have plagued this part of the Kanto plain. It is possible to book a tour underground.

• Petra

• 🌍 The 3rd Tunnel (제3땅굴). In the demilitarized border zone between the Koreas there are tunnels dug by North Korea for the purpose of secretly moving troops under the border to the South. South Korea has found four such tunnels and the third one (or rather a section of it on the South Korean side) has been turned into a tourist attraction, often visited on a tour to the DMZ. As with many places in the DMZ, photography is not allowed in the tunnel itself but it's so dark and narrow you'll not be able to get any good photos anyway. (updated Dec 2015)

• Lærdalstunnelen. (updated Mar 2023)
• 🌍 Fjell festning (Fjell fortress), Sotra island at Bergen. A second world war fortress mostly in mountain tunnels. Huge artillery was mounted on the summit. Now museum. (updated Dec 2015)
• 🌍 Andersgrotta shelter, Kirkenes, Finnmark. A makeshift bomb shelter made the people of Kirkenes during the second world war. The small town of Kirkenes was the most bombed out town during the war after about 300 air raids. (updated Dec 2015)
• 🌍 Gausta funicular (Gaustabanen), Rjukan. Small train/funicular transport through a 1000 meter horizontal tunnel and a sloping 1000 meter tunnel inside iconic Gausta summit. Built for military and telecom purposes in the 1950s, now open for tourists. (updated Dec 2015)
• Bremnes fort, Lofoten. Fort with bunkers built by German forces during second world war. Artillery has been removed. Guided tours possible. (updated Dec 2015)

• 🌍 Kola Superdeep Borehole (Кольская сверхглубокая скважина Kol'skaya sverhglubokaya skvazhina), Murmansk Oblast (between Zapolyarny and Nikel). At more than 12 km (7.5 mi) in depth, this is the deepest hole ever drilled by mankind – and indeed, it is further into the earth's crust than the Mariana Trench, the deepest point in the oceans. However, this is certainly not an abyss with a huge opening at the top; do not expect to enter, descent down or even peek into the hole. You have to decide whether it is worthwhile to drive across the tundra of a remote Arctic peninsula for hours for an urbex expedition into a derelict Soviet-era research station to see a rusty and fairly unimpressive metal lid saying 12.266 metrov in Russian Cyrillic handwriting. 
• 🌍 Moscow Metro (Moscow). One of the world's largest metro networks, in Moscow, also has some spectacular showcases of Soviet art in a select number of stations which will give you a first hand taste of the 20th century history of Russia. 
• 🌍 Saint Petersburg Metro (Saint Petersburg). Opened 1955, the system exhibits many typical Soviet designs and features exquisite decorations and artwork making it one of the most attractive and elegant metros in the world. Due to the city's unique geology, it is also one of the deepest metro systems in the world. The system's deepest station, Admiralteyskaya, is 86 m (282 ft) below ground. 
• . (updated Mar 2022)

• 🌍 Basilica Cistern (Yerebatan Sarnıcı), Yerebatan Cad. 13, Sultanahmet, Istanbul. One of the many underground cisterns of Constantinople that was in use during the Roman, Byzantine, and Ottoman periods (and perhaps the most easily visitable one). Inside is an eerie "forest" of columns, standing on an ankle deep of water. (updated Dec 2015)
• 🌍 Cappadocia. The early Christians dug numerous underground cities — complete with sleeping chambers, food storage, kitchens, wineries, and even an inn for traders — into the soft volcanic soils of the Cappadocia region, to escape raids and persecution. (updated Dec 2015)

• 🌍 Chislehurst Caves, Old Hill, Chislehurst, ☏ +44 20 8467-3264, enquiries@chislehurstcaves.co.uk. W-Su 10AM-4PM, seven days during school holidays. A seriously underlooked attraction, the caves are not in fact caves but a twenty-mile long network of passageways, carved from the chalk deep under Chislehurst. Used as a massive air-raid shelter during World War II, the Caves are now a local tourist attraction. Tours often last for an hour, were you'll learn the fascinating history as well as hear ghost and horror stories. It can also be rented as a venue. £5, concessions £3, under 5's free. 
• 🌍 Cabinet War Rooms and Churchill Museum, Clive Steps, King Charles St, London/Westminster (tube: Westminster). 09:30-18:00 daily (last admission 17:00), closed 24-26 Dec. A branch of the Imperial War Museum, the Cabinet War Rooms preserves the underground corridors and rooms from which Churchill and the cabinet directed the war against Hitler and the Nazis, maintained almost exactly as they were left in 1945. Opened in 2004, the attached Churchill Museum is the world's first permanent museum dedicated to the life and wartime achievements of Sir Winston Churchill, voted the Greatest Briton in 2002. £10, children under 16 free, seniors £8, students £8, unwaged £5, group concessions available. 
• Kelvedon Hatch Secret Bunker, ☏ +44 1277 364883. 10AM to 4PM during the week and 10AM to 5PM weekends & bank holidays. A well preserved and maintained example of a Cold-war Rotor station, and one of the deepest Cold-war Bunkers in the United Kingdom. £7.00. (updated Dec 2015)
• 🌍 The Thames Tunnel, London. Although it cannot be visited directly because it is used as part of the London Overground rail network, the Brunel Museum is housed in the former engine house on the Rotherhithe side of the Thames. A concrete raft was built near the bottom of the shaft, above the tracks, when the tunnel was upgraded for the London Overground network. This space, with walls blackened with smoke from steam trains, is accessible from Railway Avenue, and functions at times as a concert venue and occasional bar. A rooftop garden has been built on top of the shaft. The entrance hall is an exhibition space, with a staircase providing easy access to the shaft. (updated Dec 2020)
• 🌍 Williamson's Tunnels, The Old Stable Yard, Smithdown Lane, Liverpool L7 3EE, ☏ +44 151 709-6868, enquiries@williamsontunnels.co.uk. Heritage Centre Tu-Su. In the early 1800s, a Liverpool tobacco merchant, Joseph Williamson, funded the construction of an enormous labyrinth of tunnels under the Edge Hill area of Liverpool. Nobody knows his reasons for doing so though many guess it as an act of philanthropy, using his wealth to provide jobs and training for thousands of Liverpool workers. There is also a Williamson's Tunnels Heritage Centre. 
• Dudley Canal Tunnels, 501 Birmingham New Road Dudley, West Midlands, DY1 4SB, ☏ +44 121 557 6265, info@dctt.org.uk. A Boat tour into the Canal tunnels under Dudley, which were used for mineral extraction at the start of the Industrial revolution. (updated Aug 2021)
• A small number of Ice Houses associated with grand houses, remain in the UK. These were used prior to modern refrigeration to store the ice used by the house.

For less well known Underground works consider joining a specialist organization such as Subterranea Brittanica who specialize in the research of such sites.

• Cu Chi Tunnels, Cu Chi. Mainly used for military purposes (although there was also a civilian life going on as the inhabitants of the town escaped the bombings on the ground), these tunnels were started in the 1940s during the French occupation and later expanded by the FNL during the Vietnam War. Amounting to a claustrophobia-inducing system of 250 km of passages below the town, some of the former sleeping chambers, hospitals, and military headquarters are open for visits. (updated Dec 2015)
• 🌍 Vinh Moc Tunnel, Demilitarized Zone. Near the border once separating North and South Vietnam from each other, the entire population of a village found refuge in this tunnel for two years during the Vietnam War. (updated Dec 2015)

If you are permitted to take simple photos, take them. You should however be aware that taking photos in low light will require a 'fast' sensor and a fast lens. Many shots will be easiest with a wide-angle lens. Seek appropriate local advice if you wish to use a flash.