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A bunker is a defensive military fortification. Bunkers are mostly below ground, compared to blockhouses which are mostly above ground. They were used extensively in World War I and World War II. During the Cold War, massive bunker complexes were built to house both strategic (command & control) infrastructure as well as government personnel and stores for the event of a nuclear war. During that time, bunkers became a part of American culture with people building backyard fallout shelters, though these were not intended to protect against direct attacks as bunkers normally would.

Contents 1 Types 1.1 Trench bunker 1.2 Pillbox 1.3 Industrial bunker 1.4 Personal shelter 2 Design 2.1 Blast protection 2.2 Nuclear protection 2.3 General features 3 Famous installations 4 See also 5 References 5.1 Footnotes 5.2 General references 6 External links

This type of bunker or blockhouse is a small concrete box, 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 and also include top protection against aerial attack (grenades, mortar shells). The front bunker of a trench system usually includes machine guns or mortars and forms a dominant shooting post. The rear bunkers are usually used as command posts or Tactical Operations Center (TOC), for storage and as field hospitals to attend to wounded soldiers.

Many mines in France were transformed into bunkers by both the Germans and the French in World War I and World War II.

Dug-in guard posts (with loopholes through which to fire guns) and 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.^[2] They are in effect a trench firing step hardened to protect against small-arms fire and grenades and raised a little to improve the field of fire.

Their use seems to have developed during the period of the First World War when defence in depth using the Machine Gun Corps was being perfected. However, most of those seen in Britain, having been left over from the 1940 invasion scare, are designed for use by riflemen rather than for machine gunners. The concrete nature of pillboxes means that they are a feature of prepared positions and their original use is likely to have been in the Hindenburg Line. This is likely to have been the time when they acquired their incongruous English name. The Oxford English Dictionary'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 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), or they may be placed to guard strategic structures such as bridges and jetties.

Many pillboxes were built before WWII in the Czech Republic in defence against the German invasion of Czechoslovakia. None of these were actually used in the end, since the German military met no resistance when coming to the country.

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.

Experts in preparedness for war (such as Cresson Kearny, see below) recommend purchasing and stockpiling the materials for an expedient blast or fallout shelter, and then constructing it only if war appears very likely. In real wars, such materials almost immediately become unavailable as emergency construction depletes stocks. The storage needed is modest, and the materials are inexpensive in peacetime, and easy to inspect and maintain.

When a house is purpose-built with a bunker, the normal location is a reinforced below-grade bathroom with large cabinets.

Today some vendors provide true bunkers engineered to provide good protection to individual families at modest cost. 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.

Bunkers deflect the blast wave from nearby explosions 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) of overpressure, bunkers are regularly constructed to survive several hundred psi (over 10 bar). This substantially decreases the likelihood that a bomb can harm the structure.

The basic plan is to provide a structure that is very strong in physical compression. The most common purpose-built structure is a buried, steel reinforced concrete vault or arch. 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, hammocks, or bean-bag chairs to protect inhabitants from the walls and floors.

Nuclear bunkers must also cope with the underpressure that lasts for several seconds after the shock wave passes, and block radiation. Usually these features are easy to provide. The overburden (soil) and structure provide substantial radiation shielding, and the negative pressure is usually only 1/3 of the overpressure.

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 fire 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 must have two doors. Normally one door is convenient, and the other is strong. Door shafts may double as ventilation shafts to reduce digging.

In bunkers inhabited for prolonged periods, large amounts of ventilation or air conditioning 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. Ventilation openings in a bunker must be protected by blast valves. A blast valve is closed by a shock wave, but otherwise remains open. One form of expedient blast valve is tire-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 film serves quite well, because the overburden protects it from degradation by wind and sunlight.

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Famous bunkers include NORAD's underground facility at Cheyenne Mountain and the Canadian set of so-called Diefenbunkers. The Soviet Union maintained huge bunkers (one of the secondary uses of the very deeply dug Moscow Subway system was as nuclear shelters), and in Albania, Enver Hoxha dotted the country with hundreds of thousands of bunkers. Dictators and potentates like Saddam Hussein often spent massive sums building fortresses beneath their palaces. Osama bin Laden at one time was also rumoured to be hiding in massive 'underground fortresses' in Tora Bora, though these would only be natural features strengthened and extended to some degree.

General topics:

• Air-raid shelter
• Blast shelter
• Blockhouse
• British Field Defences of WWII
• Bunker buster
  • Nuclear bunker buster
• Earth sheltering
• Fallout shelter
• Flak tower
• Hesco bastion
• Observation post
• Martello tower

Specific bunkers:

• Bankstown Bunker, used as a command bunker by Douglas MacArthur during WWII (even though he never visited the site) in Sydney Australia.
• Burlington, a city-sized bunker beneath Wiltshire
• Cheyenne Mountain, the underground base of NORAD
• Führerbunker, the Berlin bunker of Adolf Hitler
• World War II line-type bunker systems
  • Atlantic Wall, coasts of Western Europe, built by Nazi Germany during WWII
  • GHQ Line, southern England, built by Great Britain during WWII
  • Maginot Line, eastern France, built by France, pre-WWII
  • Siegfried Line, western Germany, built by Germany during WWI and again pre-WWII
  • Taunton Stop Line, southwest England, built by Great Britain during WWII

• Kearny, Cresson, Nuclear War Survival Skills (1990), ISBN 094248701X (online copy)

• Bunkers or bunker systems:
  • Europe Bunker Pictures (images, locations and information, mainly Atlantikwall, also some about Cold War structures)
  • Europe WW2 bunkers in Europe (reference website about WW2 pillboxes, bunkers and other defensive structures with maps and photos)
  • Great Britain Coastal Defences North Norfolk (image collection of WW2 coastal defences)
  • Great Britain Pillboxes UK (reference website about WW2 pillboxes and other defensive structures)
  • Great Britain UK Heritage: Pillboxes (short article about "Miniature modern castles")
  • Great Britain Subterranea Britannica: Cold War (information on Cold War-era underground structures in Britain)
  • Italy Some [German] Fortifications Observed in Italy (a WWII intelligence Intelligence Bulletin report including photos)
  • Malta Military History of Malta: Pillboxes (information on WW2 fortifications on Malta)