Underground works

Humans have been digging holes and underground structures since prehistory.
As well as expanding natural caves, humans have constructed or expanded countless works of their own efforts underground from early residences of prehistory through to contemporary tubes and tunnels that support a modern lifestyle, or for disposal of its wastes.
A number of underground structures were built for warfare, be it tunnels under enemy positions or underground fortresses and anything from subsurface storage rooms through to the hardened bunkers built for a "day after" - which thankfully never arrived.
Other Underground works however were built to aid the flow of trade and people, be it road, river or rail, going underground made sense to the engineers of history when the surface way was congested or impractical to the demands. New underground structures are still being built to this day and sometimes they tunnel through the same mountain again but at a lower level, making for so called "base tunnels".
There are also the underground follys, and subterranean grottoes built by those with both the money and eccentricity to desire them.

The easiest underground works to access are those which were clearly designed as public spaces or where tourist facilities have been installed specifically to aid access by visitors.
A number of cites have vast underground public spaces, such as Japan's underground shopping malls, or Houston's tunnel system. In other cities railway stations are partly or entirely under ground. Leipzig for instance recently opened new underground through tracks beneath its multilevel shopping mall / terminus railway station.

Urban rail

Many large cites 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, 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.

Military sites

However, not all underground works are necessarily open to the traveller (even on an organized tour), It should be especially noted that 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.

For the countless underground works resulting from mining or quarrying see Mining tourism.
See also Nuclear_bunkers for cold-war bunkers.

Australia

South Australia

• Coober PedyAn opal mining town in the South Australia Outback that is almost fully underground, as this is the only way to ensure human habitation in the harsh climate.

Western Australia

• Fremantle Prison TunnelsA tunnel system in Western Australia underneath the Fremantle Prison

Bosnia and Herzegovina

• Sarajevo War Tunnel Museum

  address: 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.

Canada

• Diefenbunker - Canada's Cold War Museum

  address: Carp, Ontario
  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

• Fort Peninsula

  address: Forillon National Park, Gaspé, Quebec
  Quebec's only fully preserved World War II-era shoreline battery, Fort Peninsula was one of the three fixed defences that comprised HMCS Fort Ramsay, a naval base established in 1942 by the Canadian military to defend against Nazi U-Boat attacks, to ensure the safety of merchant vessels passing through the region, and to serve as a refuge for the British Royal Navy in the event that Hitler's forces were to successfully conquer the UK. Nineteen warships based here served a key role in waging the Battle of the St. Lawrence which saw 23 Allied vessels sunk by German subs off Canada's east coast. Today, visitors can walk through the underground corridors of the fortification and observe the vintage gun mortars and other artillery still pointed seaward, and read descriptive panels along the way that explain the strategic military importance of the Gaspé Peninsula during the Second World War. Outside, there's a pleasant seaside picnic area.

France

• Musée des Égouts de Paris

  phone: +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.
• The Mines of Paris often erroneously termed the Catacombs.

Germany

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

  address: 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.

• Berliner UnterweltenThe "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.

Italy

• 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

  address: Piazza san Gaetano
  Caves under the city center created by mining from the first Greek settlers 2000 years ago.

Japan

• 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.

Korea

• The 3rd TunnelIn 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.

Norway

• Fjell festning

  address: Sotra island at Bergen
  A second world war fortress mostly in mountain tunnels. Huge artillery was mounted on the summit. Now museum.

• Andersgrotta shelter

  address: 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.

• Gausta funicular

  address: 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.

• Bremnes fort

  address: Lofoten
  Fort with bunkers built by German forces during second world war. Artillery has been removed. Guided tours possible.

Russia

• Kola Superdeep Borehole

  address: Murmansk Oblast
  At more than 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, and do not expect to enter, descent down or even peek into the hole, so 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.

Turkey

• Basilica Cistern

  address: 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.

• CappadociaThe 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.

United Kingdom

• Chislehurst Caves

  phone: +44 20 8467-3264
  address: Old Hill, Chislehurst
  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.

• Cabinet War Rooms and Churchill Museum

  address: Clive Steps, King Charles St, London/Westminster
  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, recently voted the Greatest Briton.

• Kelvedon Hatch Secret Bunker

  phone: +44 1277 364883
  A well preserved and maintained example of a Cold-war Rotor station, and one of the deepest Cold-war Bunkers in the United Kingdom.
• The Thames Tunnel. Although this cannot be visited directly owing to it use 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. The original entrance shaft (formed using possibly the first noted 'cassion', has with recent modifications been renovated into a "venue space" and it was planned (as of 2001) to eventually house a permanent exhibition in it.

• Williamson's Tunnels

  phone: +44 151 709-6868
  address: The Old Stable Yard, Smithdown Lane, Liverpool L7 3EE
  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.

• Dover Castle

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

Vietnam

• Cu Chi Tunnels

  address: 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 Viet Cong 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.

• Vinh Moc Tunnel

  address: 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.

Photos

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/or a fast lens. Many shots will be easiest with a wide-angle lens. Seek appropriate local advice if you wish to use a flash.

(see also Urbex#Stay safe)
There are few risks associated with underground works clearly adapted for tourism or intended as accessible public spaces, and if you have any doubts as to your ability or fitness (including mental attitude), sticking to these is very strongly recommended. Sites that have organized formal tours will also have established rules and procedures which should be adhered to.
In order to stay safe when visiting less accessible underground works, It is vital to know as much as possible about the specific sites(s) you wish to visit in advance, so you can plan accordingly.
Underground works, where access, up to and including the entrance, involves tight crawls; confined space; vertical drops; sharp climbs; any expanse or mass of water or in general where a failure of any equipment is going to become a critical problem, are considered beyond the scope of accessibility for the non-specialist. For safety advice concerning visiting these caves or works, specialist advice will need to be obtained from dedicated caving and subterranean exploration organizations, familiar with the specific site concerned.
For other reasonably accessible underground works, noting the previous paragraph, which are not as well adapted for the tourist or traveller (if at all), you should seek and heed local advice even if the works concerned seems easy at a cursory glance at the surface or entrance. Contacting the sites current (or former) operator in advance, will also allow them to inform a potential visitor of any known risks.
There is some detailed advice on cave safety at this site which is also applicable to underground works but some common sense follows:
Know the works you wish to visit and your exit(s) before you enter, and if there is more than one entrance or exit, know which ones will be safe to use. Practically all responsible underground explorations are planned only after weeks or months of research.
NEVER enter underground works alone, because not only will there be no-one to get you out, but no-one will know where you are!
It is best practice to follow the example of the professionals and visit in groups of at least four. As well as lodging plans with appropriate contacts, in nearly all circumstances a surface watchman is essential whose responsibility is to contact the authorities if things go bad or a group below the surface fails to return by a specified time.
Your "smart" phone will have little or no service, below ground.
Underground works are naturally dark, and without artificial light (which in disused or abandoned works you will have to provide yourself), you will have a hard time navigating them. Don't rely on a single light source, which could fail. Spare light sources are strongly recommended.
Check the weather first! A number of Underground works can and have flooded. In wet conditions water levels can change unexpectedly, limiting or cutting off access routes or even exits. In others the removal of flood or storm water is a primary function of the work concerned, and the water level in the system can change automatically within minutes. Getting caught by rising water levels isn't worth it.
DO NOT under under any circumstances enter any underground expanse or mass of water, without having in advance sought appropriate advice. (Not only can the depth be deceptive, but the water may not be as pure as its appearance suggests). Caution should similarly be exercised in respect of moderate mud; silt; and fallen debris.
Whilst countless disused military facilities may seem abandoned, many of them are still nominally highly restricted sites, and unauthorized access to these will at the very least lead to a lengthy interrogation by the respective military authority. Many civilian facilities, are also considered sensitive by their respective current or former operators, for safety and security reasons. If in any doubt confirm your intended plans with the relevant authorities, site owners or operators well in advance.
Disused facilities are not going to be well maintained, and fixed access equipment may well have been removed at abandonment, or has subsequently decayed beyond use.
In addition to dust, the local wildlife may not appreciate being unduly disturbed.

Be respectful of the underground environment you are visiting. Ideally you should try to leave the underground environment as you found it as far as possible. No garbage and human waste should be left behind. The very presence of human beings and light can also severely change the micro-climate in the underground environment, issues like Lampenflora or plants growing due to the light and warmth of artificial light sources being a serious problem in some underground structures.
If the Underground works are not generally accessible or don't operate formal tours, discreetly and politely seek the consent of the site's owners and operators first. Not only does this express your genuine interest in any given site, but will allow those familiar with the site to advise on specific procedures, taboos or prohibitions. Some site owners and operators may also be willing to provide additional information to place the site in context. Joining a specialist group, is also recommended.
Exceptional care must be exercised where unique historic (or prehistoric) cultural heritage exists, so that it it is not lost or degraded to future generations. Some of these sites are associated with tragic historical events and others are literally the place where people died or were buried. Some underground structures may also be considered "holy ground" in a religious or secular sense, so behave accordingly.