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A military bunker is a hardened shelter, often buried partly or fully underground, designed to protect the inhabitants from falling bombs or other attacks. They were used extensively in World War I
World War I
World War I , also known as the First World War, the Great War, and the War to End All Wars, was a global military conflict which involved most of the world's great powers, assembled in two opposing alliances: the Triple Entente and the Triple Alliance...

, World War II
World War II
World War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...

, and the Cold War
Cold War
The Cold War was the continuing state of political conflict, military tension, and economic competition existing after World War II , primarily between the USSR and its satellite states, and the powers of the Western world, including the United States...

 for weapons facilities, command and control centers, and storage facilities (for example, in the event of nuclear war
Nuclear warfare
Nuclear warfare, or atomic warfare, is a military conflict or political strategy in which nuclear weapons are used. Compared to conventional warfare, nuclear warfare is vastly more destructive in range and extent of damage...

).

Trench


This type of bunker is a small concrete structure, partly dug into the ground, which is usually a part of a trench system. Such bunkers give the defending soldiers better protection than the open trench
Trench
-Agriculture:Trenches have long been used to carry water. Trenches can be used for draining purposes, leading water away from a swamp or wetland that is to be dried out. Likewise they can be used for irrigation purposes, directing water into dry areas...

 and also include top protection against aerial attack (grenades, mortar
Mortar (weapon)
A mortar is a muzzle-loading indirect fire weapon that fires shells at low velocities, short ranges, and high-arcing ballistic trajectories. It typically has a barrel length less than 15 times its caliber.- Function :...

 shell
Shell (projectile)
A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage includes large solid projectiles previously termed shot...

s). They also provide shelter against the weather.

The front bunker of a trench system usually includes machine gun
Machine gun
A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rifle bullets in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute...

s or mortars and forms a dominant shooting post. The rear bunkers are usually used as command posts or Tactical Operations Center
Tactical Operations Center
A tactical operations center is a command post for police, paramilitary, or military operations. A TOC usually includes a small group of specially trained officers or military personnel who guide members of an active tactical element during a mission...

 (TOC
Tactical Operations Center
A tactical operations center is a command post for police, paramilitary, or military operations. A TOC usually includes a small group of specially trained officers or military personnel who guide members of an active tactical element during a mission...

), for storage and as field hospital
Hospital
A hospital is an institution for health care providing patient treatment by specialized staff and equipment, and often but not always providing for longer-term patient stays....

s to attend to wounded soldiers.

Pillbox


Dug-in guard posts (with loopholes through which to fire weapons) made from concrete are also known as "pillboxes". The originally jocular name arose from their perceived similarity to the cylindrical boxes in which medical pills were once sold.. They are in effect a trench firing step hardened to protect against small-arms fire and grenades and raised to improve the field of fire.

Their use seems to have developed during the period of the First World War
World War I
World War I , also known as the First World War, the Great War, and the War to End All Wars, was a global military conflict which involved most of the world's great powers, assembled in two opposing alliances: the Triple Entente and the Triple Alliance...

 when defence in depth
Defence in depth
Defence in depth is a military strategy sometimes referred to as elastic defence or deep defence. Defence in depth seeks to delay rather than prevent the advance of an attacker, buying time and causing additional casualties by yielding space...

 using the Machine Gun Corps
Machine Gun Corps
The Machine Gun Corps was a corps of the British Army, formed in October 1915 in response to the need for more effective use of machine guns on the Western Front in World War I. The Heavy Branch of the MGC was the first to use tanks in combat, and the branch was subsequently turned into the Tank...

 was being perfected. However, most of those seen in Britain, having been left over from the 1940 invasion scare
British anti-invasion preparations of World War II
British anti-invasion preparations of World War II entailed a large-scale division of military and civilian mobilization in response to the threat of invasion by German armed forces in 1940 and 1941. The army needed to recover from the defeat of the British Expeditionary Force in France, and 1.5...

, are designed for use by riflemen rather than for machine gun
Machine gun
A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rifle bullets in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute...

ners. The concrete nature of pillboxes means that they are a feature of prepared positions. They were probably first used in the Hindenburg Line
Hindenburg Line
The Hindenburg Line was a vast system of defences in northeastern France during World War I. It was constructed by the Germans during the winter of 1916–17...

. This is likely to have been the time when they acquired their incongruous English name. The Oxford English Dictionary
Oxford English Dictionary
The Oxford English Dictionary , published by the Oxford University Press , is a comprehensive dictionary of the English language...

s earliest record of the use of the word
pillbox in connection with a defensive post is from 13 September 1917, after the German withdrawal onto the Hindenburg Line.

Pillboxes are often camouflaged
Military camouflage
Military camouflage became an essential part of modern military tactics after the increase in accuracy and rate of fire of weapons during the 19th century. Until the 20th century armies tended to use bright colors and bold, impressive designs. These were thought to daunt the enemy, foster unit...

 in order to conceal their location and to maximize the element of surprise. They may be part of a trench system, form an interlocking line of defence with other pillboxes by providing covering fire to each other (defence in depth
Defence in depth
Defence in depth is a military strategy sometimes referred to as elastic defence or deep defence. Defence in depth seeks to delay rather than prevent the advance of an attacker, buying time and causing additional casualties by yielding space...

), or they may be placed to guard strategic structures such as bridges and jetties.

Pillboxes for the Czechoslovak border fortifications
Czechoslovak border fortifications
The Czechoslovak government built a system of border fortifications from 1935 to 1938 as defensive countermeasure against the rising threat of Nazi Germany that later materialized in the German offensive plan called Fall Grün...

 were built before WWII in the Czech Republic
Czech Republic
The Czech Republic is a country in Central Europe that is sometimes considered to be Eastern European. The country borders Poland to the northeast, Germany to the west and northwest, Austria to the south and Slovakia to the east. The capital and largest city is Prague...

 in defence against the German invasion of Czechoslovakia. None of these were actually used against its intended enemy, since the German military met no resistance when invading the country because it was effectively forced to capitulate as a result of Allies annexing
Munich Agreement
The Munich Agreement was an agreement permitting German annexation of Czechoslovakia's Sudetenland. The Sudetenland were areas along borders of Czechoslovakia, mainly inhabited by Czech Germans. The agreement was negotiated at a conference held in Munich, Germany, among the major powers of Europe...

 the country's border areas and handing them to Germany, but some were used against the liberating Russian armies. The Japan
Japan
is an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

ese also made use of pillboxes in their fortifications of Iwo Jima
Battle of Iwo Jima
The Battle of Iwo Jima , or Operation Detachment, was a battle in which the United States fought for and captured Iwo Jima from Japan...

.

Artillery


Many artillery installations, especially for naval artillery
Naval artillery
Naval artillery or naval rifles refers to warship-mounted guns used in naval warfare for attacking enemy vessels, bombarding targets on shore , or for anti-structural demolition. Conversely, the term may be used as a descriptor about the naval rifles used in land batteries for anti-shipping area...

 have historically been protected by extensive bunker systems. These usually housed the crews serving the weapons, protected the ammunition against counter-battery fire, and in numerous examples also protected the guns themselves, though this was usually a trade-off reducing their fields of fire. Artillery bunkers are some of the largest individual pre-Cold War bunkers. The walls of the 'Batterie Todt' gun installation in northern France were up to 3.5 m thick, and an underground bunker was constructed for the V-3 cannon
V-3 cannon
The V-3 , also known as the Hochdruckpumpe and Fleissiges Lieschen , was a German World War II supergun working on the multi-charge principle whereby secondary charges are detonated to add velocity to a projectile.The weapon was...

.

Industrial


Typical industrial bunkers include mining sites, food storage areas, dumps for materials, data storage, and sometimes living quarters. They were built mainly by nations like Germany during World War II to protect important industries from aerial bombardment. Industrial bunkers are also built for control rooms of dangerous activities, e.g. tests of rocket engines or explosive experiments. They are also built in order to perform dangerous experiments in them or to store radioactive or explosive goods. Such bunkers also exist on non-military facilities.

Personal


When a house is purpose-built with a bunker, the normal location is a reinforced below-grade bathroom with large cabinets. One common design approach uses fiber-reinforced plastic shells. Compressive protection may be provided by inexpensive earth arching. The overburden is designed to shield from radiation. To prevent the shelter from floating to the surface in high groundwater, some designs have a skirt held-down with the overburden. It may also serve the purpose of a safe room.

Blast protection




Bunkers deflect the blast wave from nearby explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

s to prevent ear and internal injuries to people sheltering in the bunker. While frame buildings collapse from as little as 3 psi (0.2 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and rougly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the decibar , centibar , and millibar...

) of overpressure
Overpressure
The term Overpressure is applied to a pressure difference, relative to a "normal" or "ambient" pressure, in two different circumstances:* In geology: the pressure regime in a stratigraphic unit that exhibits higher-than-hydrostatic pressure in its pore structure. This phenomenon is the primary...

, bunkers are regularly constructed to survive several hundred psi (over 10 bar). This substantially decreases the likelihood that a bomb
Bomb
A bomb is any of a range of explosive devices that typically rely on the exothermic chemical reaction of an explosive material to produce an extremely sudden and violent release of energy. The word comes from the Greek word βόμβος , an onomatopoetic term with approximately the same meaning as...

 (other than a bunker buster
Bunker buster
A bunker buster is a bomb designed to penetrate hardened targets or targets buried deep underground.-World War II:In World War II the British designer Barnes Wallis, of bouncing-bomb fame, designed two bombs that would today be called bunker busters: the five tonne Tallboy and the ten tonne Grand...

) can harm the structure.

The basic plan is to provide a structure that is very strong in physical compression
Physical compression
Physical compression is the result of the subjection of a material to compressive stress, resulting in reduction of volume. The opposite of compression is rarefraction tension.- Explanation :...

. The most common purpose-built structure is a buried, steel reinforced concrete
Reinforced concrete
Reinforced concrete is concrete in which steel reinforcement bars , plates or fibers have been incorporated to strengthen a material that would otherwise be brittle.-History:...

 vault
Vault (architecture)
A Vault is an architectural term for an arched form used to provide a space with a ceiling or roof. The parts of a vault exert a thrust that require a counter resistance. When vaults are built underground, the ground gives all the resistance required...

 or arch
Arch
An arch is a structure that spans a space while supporting weight . Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-History:Arches...

. Most expedient (makeshift) blast shelters are civil engineering structures that contain large buried tubes or pipes such as sewage or rapid transit tunnels. Improvised purpose-built blast shelters normally use earthen arches or vaults. To form these, a narrow (1-2 metre) flexible tent of thin wood is placed in a deep trench (usually the apex is below grade), and then covered with cloth or plastic, and then covered with 1–2 meters of tamped earth.

A large ground shock can move the walls of a bunker several centimeters in a few milliseconds. Bunkers designed for large ground shocks must have sprung internal buildings to protect inhabitants from the walls and floors.

Nuclear protection


Nuclear bunkers must also cope with the underpressure that lasts for several seconds after the shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

 passes, and block radiation
Radiation
In physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body...

. Usually these features are easy to provide. The overburden (soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics. It is composed of particles of broken rock that have been altered by chemical and...

) and structure provide substantial radiation shielding, and the negative pressure is usually only 1/3 of the overpressure.

General features



The doors must be at least as strong as the walls. The usual design is a trap-door, to minimize the size and expense. To reduce the weight, the door is normally constructed of steel, with a fitted steel lintel and frame. Very thick wood also serves, and is more resistant to heat because it chars rather than melts. If the door is on the surface and will be exposed to the blast wave, the edge of the door is normally counter-sunk in the frame so that the blast wave or a reflection cannot lift the edge. A bunker should have two doors. Door shafts may double as ventilation shafts to reduce digging.

In bunkers inhabited for prolonged periods, large amounts of ventilation
Ventilation (architecture)
Ventilation is the intentional movement of air from outside a building to the inside. It is the V in HVAC. With clothes dryers, and combustion equipment such as water heaters, boilers, fireplaces, and wood stoves, their exhausts are often called vents or flues — this should not be confused...

 or air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area, or provide heat to an area. The cooling is done using a simple refrigeration cycle...

 must be provided in order to prevent ill effects of heat. In bunkers designed for war-time use, manually-operated ventilators must be provided because supplies of electricity or gas are unreliable. One of the most efficient manual ventilator designs is the Kearny Air Pump
Kearny Air Pump
The Kearny Air Pump is an expedient air pump used to ventilate a shelter. The design is such that a person with normal mechanical skills can construct and operate one. It is usually human powered and employed during a time of crisis...

. Ventilation openings in a bunker must be protected by blast valve
Blast valve
A blast valve is used to protect a shelter, such as a fallout shelter or bunker, from the effects of sudden outside air pressure changes. A nuclear weapon creates a shock wave, which may produce sudden pressure changes of more than an atmosphere even several miles or kilometers from the detonation...

s. A blast valve is closed by a shock wave, but otherwise remains open. One form of expedient blast valve is tyre-treads nailed or bolted to frames strong enough to resist the maximum overpressure.

If a bunker is in a built-up area, it may have to include water-cooling or an immersion tub and breathing tubes to protect inhabitants from fire storms.

Bunkers must also protect the inhabitants from normal weather, including rain, summer heat and winter cold. A normal form of rainproofing is to place plastic film on the bunker's main structure before burying it. Thick (5-mil or 0.13 mm), inexpensive polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons. Its primary use is within packaging .- Description :Polyethylene is a thermoplastic polymer consisting of long chains of the monomer ethylene...

 film serves quite well, because the overburden protects it from degradation by wind and sunlight.


Countermeasures


Bunkers can be destroyed with powerful explosives and bunkerbusting warheads. The crew of a pillbox can be killed with flamethrower
Flamethrower
A flamethrower is a mechanical device designed to project a long controllable stream of fire.Some flamethrowers project a stream of ignited flammable liquid; some project a long gas flame. Most military flamethrowers use liquids, but commercial flamethrowers tend to use high-pressure propane and...

s.

Famous installations


Famous bunkers include the post-World War I Maginot Line
Maginot Line
The Maginot Line , named after French Minister of Defense André Maginot, was a line of concrete fortifications, tank obstacles, artillery casemates, machine gun posts, and other defenses, which France constructed along its borders with Germany and Italy, in the light of experience from World War I,...

 on the French eastern border and Czechoslovak border fortifications
Czechoslovak border fortifications
The Czechoslovak government built a system of border fortifications from 1935 to 1938 as defensive countermeasure against the rising threat of Nazi Germany that later materialized in the German offensive plan called Fall Grün...

 mainly on the northern Czech boarder facing Germany (but to lesser extent all around), Fort Eben-Emael
Fort Eben-Emael
Eben-Emael was a Belgian fortress between Liège and Maastricht, near the Albert Canal, defending the Belgian-German border. Constructed in 1931–1935, it was reputed to be impregnable. But on 10 May 1940, 78 paratroopers of the German 7th Flieger landed on the fortress with gliders...

 in Belgium, World War II Führerbunker
Führerbunker
The Führerbunker was located beneath Hitler's New Reich Chancellery in Berlin, Germany...

, the V-weapon installations in Germany (Mittelwerk
Mittelwerk
Central Works was an underground World War II factory that used Mittelbau-Dora forced labor in 2 main tunnels in the Kohnstein.-Mittelwerk GmbH:...

) & France (e.g. La Coupole
La Coupole
La Coupole was a Nazi Germany bunker constructed in a former limestone quarry and planned for V-2 rocket assembly and launches. The site was bombed as a "Heavy Crossbow" target and the rockets were launched from mobile firing batteries, instead.-History:...

, and Le Blockhaus
Le Blockhaus
The Bunker of Éperlecques was built by Nazi Germany under the code name Kraftwerk Nord West to be a V-2 rocket launching bunker...

) and the Cold War installations in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 (Cheyenne Mountain
Cheyenne Mountain
Cheyenne Mountain is a mountain located on the southwest side of Colorado Springs, Colorado, US, and is home to the Cheyenne Mountain Air Force Station and its Cheyenne Mountain Directorate, formerly known as the Cheyenne Mountain Operations Center .During the Cold War and continuing to today, the...

, Site R
Site R
The Raven Rock Mountain Complex is a United States government facility on Raven Rock, a mountain in the U.S. state of Pennsylvania. It is located about 14 km east of Waynesboro, Pennsylvania, and 10 km north-northeast of Camp David, Maryland. It is also called the Raven Rock Military...

, and The Greenbrier
The Greenbrier
The Greenbrier is a Mobil four star and AAA Five Diamond Award winning luxury resort located in the town of White Sulphur Springs in Greenbrier County, West Virginia, United States....

), United Kingdom (Burlington), Canada (Diefenbunker
Diefenbunker
Emergency Government Headquarters are nuclear fallout bunkers built by the Government of Canada at the height of the Cold War during the infancy of the ICBM threat...

), and Australia (Bankstown Bunker
Bankstown Bunker
The Bankstown Bunker is a disused RAAF operations facility, located on the corner of Marion and Edgar Street, Bankstown, New South Wales....

). The Soviet Union
Soviet Union
The Union of Soviet Socialist Republics was a constitutionally socialist state that existed in Eurasia from 1922 to 1991. The name is a translation of the , tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik, abbreviated СССР, SSSR. The common short name is Soviet Union, from , Sovetskiy Soyuz...

 maintained huge bunkers (one of the secondary uses of the very deeply dug Moscow Subway system was as nuclear shelters). A number of facilities were constructed in China
China
China is a cultural region, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....

, such as Beijing
Beijing
Beijing is a metropolis in northern China and the capital of the People's Republic of China...

's Underground City
Underground City (Beijing)
The Underground City , also known as Dixia Cheng, is a bomb shelter comprising a network of tunnels located beneath Beijing, China, which has since been transformed into a tourist attraction. It has been called the Underground Great Wall because it was built for the purpose of military defense...

 and Underground Project 131
Underground Project 131
Underground Project 131 is a system of tunnels in China's Hubei province constructed in the late 1960s and the early 1970s to accommodate the Chinese military command headquarters in case of a nuclear war...

 in Hubei
Hubei
' is a central province of the People's Republic of China. Its abbreviation is 鄂 , an ancient name associated with the eastern part of the province since the Qin Dynasty. The name Hubei means "north of the lake", referring to Hubei's position north of Lake Dongting...

; in Albania
Albania
Albania , officially the Republic of Albania , is a Mediterranean country in South Eastern Europe. It is bordered by Montenegro to the north, Kosovo to the northeast, Macedonia to the east and Greece to the south-east...

, Enver Hoxha
Enver Hoxha
' was the Communist leader of Albania from the end of World War II until his death in 1985, as the First Secretary of the Party of Labour of Albania...

 dotted the country with hundreds of thousands of bunkers.

Osama bin Laden
Osama bin Laden
Osama bin Mohammed bin Awad bin Laden is a member of the prominent Saudi bin Laden family and one of the founders of the terrorist organization al-Qaeda, best known for the September 11 attacks on the United States and its associations with numerous other mass-casualty attacks against...

 at one time was also rumoured to be hiding in massive 'underground fortresses' in Tora Bora
Tora Bora
Tora Bora , known locally as Spīn Ghar, is a cave complex situated in the White Mountains of eastern Afghanistan, in the Pachir Wa Agam District of Nangarhar province, approximately 50 km west of the Khyber Pass and 10 km north of the border of the Federally Administered Tribal Areas in...

, though these would only be natural features strengthened and extended to some degree.