Eighty years after the Allied powers began bombing Europe to defeat Nazi Germany in WWII, some 6,000 unexploded bombs are still estimated to be lodged below ground in Slovenia, posing a threat.

Historian Sašo Radovanovič, a co-author of the monograph Bombing Slovenia 1944-1945, has been speaking up about the danger of unexploded ordnance (UXO) and the need to map danger zones.

He is preparing an exhibition on what he says is a neglected topic in the country at the Pivka Park of Military History, one of the most popular museums in Slovenia.

Based on data and recordings of the bombings, Radovanovič estimates that around 60,000 bombs were dropped on Slovenia and around 6,000 of them never exploded.

Most bombs dropped on Maribor

Nearly half of the bombs are buried in and around Maribor, Slovenia's most heavily bombed city. Held by the Nazi Germany at the time, it was the site an important plant for aircraft components.

In addition to Maribor, bombs are most likely to be found around Nova Gorica, Ljubljana, Jesenice, Celje and the Cerklje ob Krki airbase, as well as under the entire railway infrastructure.

Earlier this year a 250-kilo WWII-era aerial bomb discovered in Maribor by construction workers was defused just three weeks after three similar bombs were deactivated in Nova Gorica. These were found as technicians scanned a site on the border with Italy in preparation for an upgrade of the Nova Gorica train station.

Time-delay fuse bombs most dangerous

The most dangerous bombs are those equipped with a time-delay fuse, which represent around 10% of the bombs dropped by Americans and the British in 1944 and 1945.

The time-delay fuse is a part of the bomb with aluminium sheets and acetone. Once acetone runs over the aluminium, it melts, triggering the firing pin and setting off the detonator.

"Some bombs didn't explode because they made a J-shaped curve in the ground and are turned nose-side up, so the acetone did not run over the aluminium sheets. But both aluminium sheets and detonators are decaying fast because of corrosion and when they disintegrate, there will be an explosion. It's only a matter of time," Radovanovič told the Slovenian Press Agency.

Also dangerous are incendiary bombs with white phosphorous and magnesium. Once their metal casing decays, phosphorous will come into contact with air and start burning. "We do not know exactly what will happen," says Radovanovič.

Call to map danger zones

Many Western European cities have already identified areas of intensive bombings. "First they look at old aerial photographs and mark the spots where the bombs fell. Germans and Austrians found that such a marking means a 90% probability that a bomb is there," the historian explains.

In the next phase bomb technicians go out in the field to find the bomb and deactivate it.

Radovanovič is urging a similar systematic approach to finding unexploded bombs in Slovenia.

In Slovenia the Protection Against Natural and Other Disasters Act and UXO protection regulations determine UXO protection measures.

The Civil Protection and Disaster Relief Administration has a UXO protection unit, which is responsible for the deactivation and removal of unexploded bombs.

The Defence Ministry's strategic communication service says that to be able to draft changes to the legislation, a lot more has to be done in obtaining data on the danger and registering sites with UXO.

In the first phase, documents on bombings would have to be obtained to create a general analysis. Then high-resolution aerial photographs of the bombings would have to be obtained, which would have to be included in a coming budget as it would require a lot of resources, the ministry service says.

This year Radovanovič was able to look into the archive of the US Air Force Historical Research Agency, which is keeping around seven million items.

He found around 100,000 pages potentially useful for Slovenia. "All the data we did not have before is hiding right here," he says.