Why did the World War II bomb suddenly explode 80 years after it fell?

A bomb dropped in the later stages of World War II, which was planned to explode about an hour to a week after impact, did not explode at Miyazaki Airport in Miyazaki for nearly 80 years.

The bomb, which created a crater in the former military airport taxiway when it exploded underground on Oct. 2, was a delayed detonation bomb dropped by U.S. forces, according to the Department of Defense.

In addition, many similar cases previously confirmed in Japan also involved bombs fitted with time-delay fuses.

WHY IS THERE A DELAY IN THE EXPLOSION?

Most delayed-detonation bombs dropped on Japan by U.S. forces during the war were timed to explode one hour to six days after hitting the ground.

When such a bomb is dropped, wind pressure causes propellers attached to the base to rotate and break a glass container filled with corrosive liquid.

The liquid then begins to melt the celluloid material used as part of the stopper, causing explosion after a certain period of time depending on the thickness of the celluloid and other factors.

“Ordinary fuse bombs do not explode unless they are subjected to a serious impact, but unexploded time bombs are unstable because their triggers are powered by small fragments,” said Tatsuya Yamamoto, a member of a civilian group formed to keep weather records. raids and battle damage, but also knowledgeable about bombs.

It is also possible for unexploded bombs to detonate if the glass container breaks due to vibration or other factors, or if the celluloid material and other parts deteriorate over time.

LEFT IN KYUSHU, TOKYO

There were about 20 cases of spontaneous unexpected bomb explosions after the war, especially in the Kyushu region and western Tokyo, according to a Tokyo-based association specializing in the detection of unexploded ordnance.

Tsuyoshi Tanaka, head of the geological research department at Nippon Geophysical Prospecting Co., headquartered in Tokyo and conducting searches for unexploded ordnance, said time-delayed detonators may be responsible for spontaneous explosions.

“There are no such cases in Osaka, Nagoya and other places where large numbers of bombs were dropped, but they did occur in places where time-delayed fuse bombs were dropped,” he said.

The areas where time-delayed bombs were dropped were closely related to their intended use.

They were used three times during air raids targeting military aircraft factories in western Tokyo in April 1945.

Time-delay bombs were used to delay the explosion to prevent smoke from explosions from covering areas and to help pursuing attackers hit their targets.

Of the approximately 20 spontaneous explosions that occurred, nine occurred between 1946 and 1982 in western Tokyo and surrounding areas.

ASSISTANCE TO OPERATIONS IN OKINAWA

According to records of air strikes conducted by B-29s, U.S. forces in the Kyushu region primarily targeted airfields operated by imperial powers in support of their operations in Okinawa Prefecture.

After US forces landed on Okinawa’s main island in April 1945, they were heavily damaged by kamikaze suicide air strikes.

US forces decided to use time bombs to neutralize airfields in the Kyushu region where pilots were flying suicide missions.

After the bombs were dropped on airports, they seeped deep into the ground and there was no way of knowing when they would explode.

While this caused delays in the repair work, in some cases the runways were destroyed again by the explosion of bombs after the repairs were completed.

According to US military documents, they achieved great results with bombings of airfields and facilities on Kyushu.

The U.S. military said the bombardments with time-delayed bombs tormented Japanese forces and prevented them from establishing the area as a staging post to counter their campaign on Okinawa.

Bombing targets were also expanded to include an airfield at Matsuyama and other facilities in the southern and western parts of the Shikoku region.

The documents also show that 250-kilogram bombs equipped with a time-delayed fuse were dropped on the premises of Miyazaki Airport, an airfield for the Imperial Navy of Japan.

Non-self-exploding bomb explosions have been confirmed in Kyushu, including Saiki in Oita Prefecture in 1969, Oita in 1992 and 1994, Hyuga in Miyazaki Prefecture in 1967, 1972 and 1975, and Kirishima in Kagoshima Prefecture in 1996.

Each city was home to an airport or aircraft factory operated by imperial powers, which was repeatedly targeted with time-delayed bombs in air raids.

Meanwhile, no US military records have been found indicating that time-delayed munitions were dropped on the main island of Okinawa.

Approximately 500 cases of unexploded bombs are investigated every year in the province where the largest land battle during the war took place.

However, there was a single confirmed case of spontaneous explosion at Nishihara in 1981.

In the municipality’s public relations magazine published at the time, it was stated that a mortar shell found in a construction site was abandoned and spontaneously burned as a result of the yellow phosphorus inside coming into contact with the air.

“Given that unexploded bombs equipped with time-delayed fuses remain near airports and other important infrastructures, I think the central government should proactively conduct investigations,” Yamamoto said. he said.

He added: “Care needs to be taken not only with unexploded ordnance containing time-delayed fuses, but also with all other bombs as there is a possibility that explosive charges may deteriorate and become easier to ignite.”