KITA-KYUSHU--After a short countdown, an explosion ripped through the calm water in the Kanmon Straits between the main islands of Honshu and Kyushu.

White splashes appeared on the surface, followed immediately by a 100-meter-high column of water.

A rumbling noise roared across the area, rocking a Maritime Self-Defense Force minesweeper about 500 meters from the blast.

In a two-step process on Dec. 14, the MSDF vessel cleared a mine from World War II.

“We managed to dispose of it without fail,” said Lt. Cmdr. Akira Ito of the 43rd Mine Warfare Force.

It was the first explosive ordnance disposal (EOD) operation carried out in the Kanmon Straits in nine years.

The still-dangerous weapon may have remained hidden had it not been for a wind farm project.

OPERATION STARVATION

During the closing days of World War II, the U.S. military dropped mines in the Kanmon Straits as part of “Operation Starvation” to disrupt Japan’s logistics in the strategic transport location.

According to the MSDF’s Sub Area Activity (SAA) Shimonoseki in Yamaguchi Prefecture, U.S. forces deployed about 5,000 mines in the straits between March and August 1945, or nearly half of the 12,000 U.S. mines planted across Japan.

The SAA Shimonoseki has continued minesweeping duties in the Kanmon Straits since it was called the Shimonoseki Naval Coastal Defense Group of the Imperial Japanese Navy during the war.

After the war, mine disposal operations were conducted under the jurisdictions of the Ministry of Demobilization and the Japan Coast Guard.

The SAA Shimonoseki has disposed of about 3,500 mines, most of them in the 20 years after the war.

FEW RECENT MINESWEEPING OPERATIONS 

A sea mine about 2 meters long and 60 centimeters in diameter is displayed in the SAA Shimonoseki’s reference room. It weighs about 700 kilograms and can contain up to 400 kg of gunpowder.

This type of mine detonates when it detects a magnetic field disturbance caused by an approaching vessel.

U.S. B-29 bombers loaded with seven to nine of these mines dropped them into the seas of Japan. For that reason, mines have been found in straight lines in many areas.

The mines damaged about 150 large ships in and around the Kanmon Straits during and after the war.

However, very few mines have been found there in recent years. Only one minesweeping operation was conducted each in 2012 and 2014.

OFFSHORE WIND FARM

One naval mine was found near and another within the Kanmon Straits in 2023. The first one was destroyed in an EOD operation in September.

They were discovered during development of the Kita-Kyushu Hibikinada Offshore Wind Farm off the city’s Wakamatsu Ward.

Under the project, 25 windmills built on the seafloor will start operating in fiscal 2025.

Magnetic studies conducted between March and November 2023 for the windmill foundations and submarine cables detected anomalies in many spots over a 2,700-hectare area.

Divers alerted the Japan Coast Guard about possible explosives in a zone running about 10 kilometers north to south and 11 km east to west.

When the MSDF conducted follow-up surveys, it found the two U.S. naval mines as well as 19 shells and bombs believed to have been discarded by the Imperial Japanese Navy.

Members of the wind farm project were not surprised by the findings.

Kita-Kyushu-based Hibiki Wind Energy Co., which primarily runs the project, said mines have been found in the area in the past, and the MSDF has informed the company about ordnance remaining from the war.

“We allocated time needed to carry out magnetic surveys when we drew up the plan so that the construction period would not be affected,” a project member said.

An official at the city’s Crisis Management Department added: “We made efforts to work together and dispose of (the mines) early so that the project wouldn’t fall behind schedule.”

DETONATION RISK LOW

Several hundred wartime mines are believed to rest on the seafloor in the Kanmon Straits and its surroundings.

“After I heard about the offshore wind farm, I was expecting to find some mines,” said Takayuki Fujii, a warrant officer stationed at the SAA Shimonoseki. “We must be prepared to find more if there are more wind farms to be built.”

However, he said there is only a 0.1 percent chance for the naval mines to explode because they have been deteriorating for decades.

But he warned that the explosives still retain their properties.

“We can’t really say there is a zero possibility, and we will dispose of them in safe places underwater,” the officer said.