A loud explosion in a field startled residents in the town of Limburg in western Germany on Sunday. The blast occurred in the middle of the night and was large enough to register a minor tremor of 1.7 on the Richter scale, according to local media.



No injuries were reported, but the mystery explosion left a crater that measured 10 meters (33 feet) wide and 4 meters deep.

Bomb specialists from the state of Hesse determined on Monday with "a probability bordering on certainty" that it was a WWII-era unexploded bomb that caused the blast.

Prior to the news release, residents were puzzled and confused by the crater, with some online speculating that it had been caused by a meteorite.

But Rüdiger Jehn, of the European Space Agency, told German newspaper Frankfurter Neue Presse that this was false. "A great deal of heat is released during an asteroid impact," the ESA expert said, adding that no evidence of heat or melting could be seen from the crater footage.

Read more:Hot weather exposes World War II munitions in German waters

A rare self-detonation

The real culprit was an aerial bomb, which was buried at a depth of at least 4 meters, weighed 250 kilograms (550 pounds) and had a chemical detonator, investigators said. Authorities confirmed that the bomb had exploded by itself, without any external trigger.

Wolfgang Spyra, a professor and engineer at the Brandenburg University of Technology in Cottbus, told DW that a self-detonation of a WWII-era bomb is very rare in Germany and only occurs once or twice per year.

"In the case of bombs with chemical detonators, the materials of the detonator decompose over a long period of time," Spyra said. The decomposition of the detonator was the cause that authorities cited for the ignition.

Read more: WWII munitions: Time bombs at the bottom of the Baltic Sea

Watch video 04:01 Share WWII bombs cast shadow over marine environment Send Facebook google+ Whatsapp Tumblr linkedin stumble Digg reddit Newsvine Permalink https://p.dw.com/p/2UJm0 WWII bombs cast shadow over marine environment

Aging bombs a threat

Limburg city spokesman Johannes Laubach told German news service NTV that finding a WWII bomb in the area was not at all surprising. "A former railway depot made this area a bombing target at the end of the war," he explained.

WWII ended more than 70 years ago, yet finding unexploded bombs in Germany is still a frequent occurrence.

Two unexploded bombs were discovered on Monday in the central German town of Giessen, prompting the temporary evacuation of some 2,500 people. Earlier this month, an unexploded device was defused in a busy area of central Berlin.

"The most frequently found explosives are cluster bombs, air bombs and bombs with incendiary materials, such as white phosphorous," Spyra said.

He warned that as bombs age and decay, they become more dangerous. This applies to both chemical detonator bombs like the one found in Limburg and conventional bombs, which tend to experience long-term corrosion.

Over time "this corrosion process leads to the weakening of the safety mechanisms within the bombs," Spyra explained.

A spokesman for the arms and munitions disposal service for the cities of Cologne and Düsseldorf echoed Spyra's assessment, telling DW that unexploded bombs in the ground "will not become less dangerous over time."

Defusing bombs – is there a safer way?

"Changes such as corrosion are visible from the outside, but any changes within the explosive ordnance cannot be detected" as the bombs are exposed to environmental changes, the spokesman said.

Between 1940 and 1945, some 2.7 million tons of bombs were dropped on Europe by US and British forces and half of them landed in Germany. Half of those that were dropped on Germany landed in North Rhine-Westphalia, the country's most populous state today.

Of the roughly quarter million bombs that did not explode, thousands are still hidden underground all over Germany.

Unexploded ordnance in Germany - a legacy of the Allied Forces What is unexploded ordnance? Unexploded ordnance (UXO or sometimes also abbreviated to UO), unexploded bombs (UXBs), or explosive remnants of war (ERW) are explosive weapons such as bombs, shells, grenades, land mines, naval mines and cluster munitions that did not explode when they were deployed. Unexploded ordnance still poses the risk of detonation, even decades after they were used or discarded.

Unexploded ordnance in Germany - a legacy of the Allied Forces Why does Germany have a bomb problem? Between 1940 and 1945, US and British forces dropped 2.7 million tons of bombs on Europe. Half of those bombs targeted Germany. Experts estimate that close to a quarter of a million bombs did not explode due to technical faults. Thousands of these bombs are still hidden underground, sometimes a few meters down and sometimes just below the surface.

Unexploded ordnance in Germany - a legacy of the Allied Forces How big is the issue? The industrial Ruhr area and the Lower Rhine region were heavily bombed, as were the cities of Dresden, Hamburg and Hanover. So this is where most of the unexploded ordnance is found. Bombs are usually unearthed during construction work or are discovered during the examination of historical aerial images. Experts say it could still take decades to clear all of the remaining unexploded ordnance.

Unexploded ordnance in Germany - a legacy of the Allied Forces What happens when an unexploded bomb is found? When confronted with the discovery of an UXO, UO or a UXB, bomb disposal experts have to decide whether to defuse it or to carry out a controlled explosion. Many have lost their lives on the job. German authorities are under pressure to remove unexploded ordnance from populated areas. Experts argue that the bombs are becoming more dangerous as time goes by due to material fatigue.

Unexploded ordnance in Germany - a legacy of the Allied Forces How many bomb disposal experts have died? Eleven bomb technicians have been killed in Germany since 2000, including three who died in a single explosion while trying to defuse a 1,000-pound bomb on the site of a popular flea market in Göttingen in 2010.

Unexploded ordnance in Germany - a legacy of the Allied Forces Which was the biggest evacuation? A 1.8-ton bomb dropped by Britain’s Royal Air Force (RAF) was found in the city center of Augsburg on December 20, 2016. The find prompted a large-scale bomb disposal operation and consequently the evacuation of over 54,000 people on December 25. To date, this remains the biggest evacuation for the removal of World War II unexploded ordnance in Germany.

Unexploded ordnance in Germany - a legacy of the Allied Forces What’s the latest? Authorities conducted another big bomb disposal operation in May 2017, with 50,000 residents in the northwestern city of Hanover forced to evacuate their homes. Thirteen unexploded ordnances from the 1940s were removed. Hanover was a frequent target of Allied bombing in the latter years of the war. On October 9, 1943, some 261,000 bombs were dropped on the city. Author: Aasim Saleem



Each evening at 1830 UTC, DW's editors send out a selection of the day's hard news and quality feature journalism. You can sign up to receive it directly here.