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Unexploded ordnance (or UXOs/UXBs, sometimes acronymized as UO) are explosive weapons (bombs, bullets, shells, grenades, land mines, naval mines, etc.) that did not explode when they were employed and still pose a risk of detonation, potentially many decades after they were used or discarded.
In the United States UXO, as deﬁned in 10 USC 101(e)(5), is one of three categories of military munitions that may pose unique explosives safety risks that fall under the broader term "Munitions and Explosives of Concern" (MEC). The other two categories of MEC are Discarded Military Munitions (DMM), as deﬁned in 10 USC 2710(e)(2) and Munitions Constituents or MC (e.g., TNT, RDX), as deﬁned in 10 USC 2710(e)(3), that is present in high enough concentrations to pose an explosive hazard.
Unexploded ordnance from at least as far back as the American Civil War still poses a hazard worldwide, both in current and former combat areas and on military firing ranges. A major problem with unexploded ordnance is that over the years the detonator and main charge deteriorate, frequently making them more sensitive to disturbance, and therefore more dangerous to handle. There are countless examples of civilians tampering with unexploded ordnance that is many years old - often with fatal results. Believing it to be harmless they handle the device and it explodes, killing or severely injuring them. For this reason it is universally recommended that unexploded ordnance should not be touched or handled by unqualified persons. Instead, the location should be reported to the local police so that Bomb disposal or Explosive Ordnance Disposal (EOD) professionals can render it safe.
Although professional EOD personnel have expert knowledge, skills and equipment, they are not immune to misfortune because of the inherent dangers: in June 2010, construction workers in Göttingen, Germany discovered an Allied 500 kilograms (1,100 lb) bomb dating from World War II buried approximately 7 metres (23 ft) below the ground. German EOD experts were notified and attended the scene. Whilst residents living nearby were being evacuated and the EOD personnel were preparing to disarm the bomb, it detonated, killing three of them and injuring 6 others. The dead and injured each had over 20 years of hands-on experience, and had previously rendered safe between 600 and 700 unexploded bombs. The bomb which killed and injured the EOD personnel was of a particularly dangerous type because it was fitted with a delayed-action chemical fuze, which had become highly unstable after over 65 years underground.
A dramatic example of MEC threat is the wreck of the SS Richard Montgomery off the coast of Kent, which still contains 3000 tons of munitions. When a similar World War II wreck, the Polish Kielce exploded in 1967, it produced an earth tremor measuring 4.5 on the Richter scale.
In the Ardennes region of France, large-scale citizen evacuations were necessary during MEC removal operations in 2001. In the forests of Verdun French government "démineurs" working for the Département du Déminage still hunt for poisonous, volatile, and/or explosive munitions and recover about 900 tons every year. The most feared are corroded artillery shells containing chemical warfare agents such as mustard gas. French and Flemish farmers still find many UXOs when ploughing their fields, the so-called "iron harvest".
Thousands of UXOs from the Second World War are still uncovered each year in Germany. The daily average is 15, most of them aerial bombs. Concentration is especially high in Berlin, where also lots of artillery shells and smaller munitions from the Battle of Berlin are uncovered each year. While most cases only make local news, one of the more spectacular finds in recent history was an American 500-pound aerial bomb discovered in Munich on 28 August 2012. As it was deemed too unsafe for transport, it had to be exploded in situ, shattering windows over a wide area of Schwabing and causing structural damage to several homes despite precautions to minimize damage. In 2011, a 1.8-tonne RAF bomb from in the Second World War was uncovered in Koblenz on the bottom of the Rhine River after a prolonged drought. It caused the evacuation of 45,000 people from the city and was called "the biggest bomb-related evacuation in Germany's post-war history". One of the largest individual pieces ever found was an unexploded 'Tallboy' bomb uncovered in the Sorpe Dam in 1958.
Laos has the dubious distinction of being the world's most heavily bombed nation. During the period of the American Vietnam War, over half a million bombing missions dropped more than 5 million tons of ordnance on Laos, most of it anti-personnel cluster bombs. Each cluster bomb shell contained hundreds of individual bomblets, "bombies", about the size of a tennis ball. An estimated 30% of these munitions did not detonate. Ten of the 18 Laotian provinces have been described as "severely contaminated" with artillery and mortar shells, mines, rockets, grenades, and other devices from various countries of origin. These munitions pose a continuing obstacle to agriculture and a special threat to children, who are attracted by the toylike devices.
Some 288 million cluster munitions and about 75 million unexploded bombs were left across Laos after the war ended. From 1996-2009, more than 1 million items of UXO were destroyed, freeing up 23,000 hectares of land.
In the aftermath of the 2006 war between Israel and Lebanon, it is estimated that southern Lebanon is littered with one million undetonated cluster bombs  - approximately 1.5 bombs per Lebanese inhabitant of the region, dropped by Israeli Defense Forces in the last days of the war. 
UXO is standard terminology in the UK, although in artillery, especially on practice ranges, an unexploded shell is referred to as a blind, and during the Blitz in World War II an unexploded bomb was referred to as a UXB.
Most current UXO risk is limited to areas, mainly in London and Portsmouth, that were subject to the Blitz and to land used by the military to store ammunition or to train on. According to the Construction Industry Research and Information Association (CIRIA), from 2006 to 2009, over 15,000 items of ordnance were found in construction sites in the UK. Most notably, 1000 homes were evacuated in Plymouth in April 2009 when a Second World War bomb was discovered, and in June 2008 a 1 000 kg bomb was found in Bow in East London. CIRIA have now published "Unexploded Ordnance (UXO) - a guide for the construction industry" to provide advice on assessing the risk posed by UXO.
The burden of Explosive Ordnance Disposal in the UK is split between Royal Engineers Bomb Disposal Officers, Royal Logistic Corps Ammunition Technicians in the Army, Clearance Divers of the Royal Navy and the Armourers of the Royal Air Force. The Metropolitan Police is the only force not to rely on the Ministry of Defence, although they generally focus on terrorist devices rather than unexploded ordnance and will often call military teams in to deal with larger and historical bombs.
Currently used training areas, such as Salisbury Plain in Wiltshire, also contain UXO. The Central Impact Area has been used by the Royal Artillery since the beginning of the 20th century. The percentage of live UXO to inert practice rounds is probably quite low, but it is impossible to distinguish between the two until they are uncovered. Some rounds will lie near the surface, some 10m down in the chalk. Because of its use it remains untouched by modern intensive farming and is thus part of the last chalk grassland in the UK (and probably Europe) and the whole area is designated as an SSSI (Site of Special Scientific Interest). Additionally the area also contains great numbers of archeological sites, which although occasionally damaged by impact, are for the most part intact. Should at some future date the army decide to stop training, and give public access, to clear the area would take hundreds of years, cost millions of pounds and destroy both the SSSI and the archaeology. It will thus remain an Out of Bounds area for time immemorial. Even cutting of scrub to stop invasion of the grass lands to retain its SSSI condition has to be carried out in an armoured mine-clearing tractor fitted with cutting flails that pass over the surface (not into it as with mine clearance) and if as often happens in dry summers, it catches fire, it has to be left to burn out (often shells will explode if near the surface) and the unit causing the fire has to stay and provide fire watchers to ensure the fire does not spread outside the impact area.
As recently as December 2007, construction areas outside Orlando, Florida discovered UXO in new development areas and had to halt construction efforts. Other areas nearby, including UXO in the Indian River Lagoon thought to be left from live bombing runs performed during World War II by pilots from nearby DeLand Naval Air Station, have long been avoided by local boaters for fear of accidentally striking UXO as they motor by.
According to US Environmental Protection Agency documents released in late 2002, UXO at 16,000 domestic inactive military ranges within the United States pose an "imminent and substantial" public health risk and could require the largest environmental cleanup ever, at a cost of at least US$14 billion. Some individual ranges cover 500 square miles (1,300 km2), and, taken together, the ranges comprise an area the size of Florida.
In addition to the obvious danger of explosion, buried UXO also entails the risk of environmental contamination. In some heavily used military training areas, munitions-related chemicals such as explosives and perchlorate (a component of pyrotechnics and rocket fuel) can enter soil and groundwater. A prominent example exists at the Massachusetts Military Reservation (MMR) on Cape Cod, Massachusetts (USA), where decades of artillery training has contaminated the only drinking water for thousands of surrounding residents. An expensive UXO recovery effort is under way there.
UXO on US military bases has also caused problems for transferring and restoring Base Realignment and Closure (BRAC) land. The Environmental Protection Agency's efforts to commercialize former munitions testing grounds are complicated by UXO, making investments and development risky.
UXO cleanup in the US involves over 10 million acres (40,000 km2) of land and 1,400 different sites. Estimated cleanup costs are tens of billions of dollars. It costs roughly $1,000 to demolish a UXO on site. Other costs include surveying and mapping, removing vegetation from the site, transportation, and personnel to manually detect UXOs with metal detectors. Searching for UXOs is tedious work and often 100 holes are dug to every 1 UXO found. Other methods of finding UXOs include digital geophysics detection with land and airborne systems.
During World War I, the US Chemical Corps was established at American University, based in the University's McKinley Building. After the war, many toxic chemicals and weaponry were buried in or around the Northwest DC community where American is located. Excavations in the area are ongoing after significant discoveries were made in 2010. 
Although comparatively rare, unexploded ordnance from the American Civil War is still occasionally found and is still deadly 150 years later. In 2008, for example, Civil War enthusiast Sam White was killed when a naval shell he was attempting to disarm exploded. 
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In cases of unexploded subsoil ordnance a remote investigation is done by visual interpretation of available historical aerial photographs. Modern techniques can combine geophysical and survey methods with modern electromagnetic and magnetic detectors. This provides digital mapping of UXO contamination with the aim to better target subsequent excavations, reducing the cost of digging on every metallic contact and speeding the clearance process. Magnetometer probes can detect UXO and provide geotechnical data before drilling or piling is carried out.
Currently in the U.S., the Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) Department of Defense programs fund research into not only the detection, but also discrimination of UXO from scrap metal. Much of the cost of UXO removal comes from removing non-explosive items that the metal detectors have identified, so improved discrimination is critical. New techniques such as shape reconstruction from magnetic data and better de-noising techniques (to name a few) will prove invaluable to reducing cleanup costs and enhancing recovery.
UXO or UXBs (as they are called in some countries - unexploded bombs) are broadly classified into buried and unburied. The disposal team carries out reconnaissance of the area and determines the location of the ordnance. In case it is unburied it may be dug carefully and disposed. But if the bomb is a buried one then it becomes a huge task. A team is formed to find the location of bomb using metal detectors and then the earth is dug carefully.
According to the US Army Environmental Command (AEC) UXO has accumulated from military training activities over the years at approximately 1,700 Formerly Used Defense Sites (FUDS), 25 Base Realignment and Closure Commission (BRAC) sites, and a number of active installations covering millions of acres.
By presidential executive order the US Armed Forces are mandated to buy "green ammunition" for use at their training ranges. Green ammunition is non-dud producing since they contain no explosives (that is, other than the propellant) and non-toxic, reducing cleanup costs and environmental risks. Environmentally sound training rounds come in 5.56 caliber and 40 mm high and low-velocity training cartridges for grenade machine guns and under-barrel grenade launchers.
Currently, the US army uses M918 40 mm cartridges, made to a pyrotechnic design from the 1970s that contains heavy metals in the fuze and potassium perchlorate in the payload. It has a fuze failure rate of 3%-8%. The US Army continues to use the M918 and M385 cartridges, favoring a "mixed-belt" approach to reducing duds and toxic leaching, however the M918 cannot be used in dry weather because of potential range fires.
US defense forces are now testing the 40 mm MK281 cartridge, Non-Dud Producing (NDP) and non-toxic training cartridge for the MK19. In 2006, the US Marine Corps signed a US$61 million 5-year contract with the Rheinmetall Group. The National Training Center (NTC) in Fort Irwin, California is partially integrating the MK281 into its operations. The U.S. Center for Health Promotion and Preventative Medicine at the Aberdeen Proving Ground, Maryland approved the MK281.
Given that the military uses 6-10 million training rounds per year, this leads to a substantial amount of UXO, leading to mandates from the Department of Defense and Environmental Protection Agency to phase out this older training ammunition. This transition is dependent on the US military-industrial base becoming ready to design and manufacture necessary amounts of green ammunition.
As of 2009, there are no US manufacturers to provide the necessary supply of green ammunition, rendering the Department of Defense's mission to phase out older ammunition at odds with its second mission to buy designs and material from US suppliers. Many small US ammunition manufacturers, many associated with the rapid creation of WWII Army Ammunition Plants, have not invested in green ammunition R&D.
"Green" also refers to the manufacturing process of ammunition. US Army programs at Picatinny Arsenal are researching methods of reducing volatile organic compounds and ozone depleting compounds during the manufacturing process. Attempts are also being made to reduce the amount of hazardous materials in the actual ammunition.
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