WWII Mines Remain in Japan’s Waters; Postwar Disposal Operations Continue

By Yohei Kano / Yomiuri Shimbun Staff Writer

1:00 JST, October 3, 2023

A sea mine that is believed to have been one of many scattered by the U.S. military at the end of World War II was blown up off the coast of Kitakyushu on Sept. 25.

The U.S. military is believed to have laid more than 10,000 mines in ports and straits in Japan to disrupt sea lanes. Although most of them have been removed, the one blown up off the coast of Kitakyushu was found on the seabed recently during a survey conducted as part of an offshore wind power generation project.

Disposal operations still in are process in the sea even 78 years after the end of the war.

On the morning of Sept. 25, a countdown of five seconds until an explosion started aboard the Maritime Self-Defense Force’s minesweeper Ukushima, which was moored about 4 kilometers north off the coast of Wakamatsu Ward, Kitakyushu. At 11:15 a.m., with the sound of an explosion, a gray column of water rose to a height of about 100 meters along with sand and other debris from the seafloor. This writer was aboard the minesweeper, which was about 300 meters away, but felt an impact that was large enough to shake the deck.

According to the MSDF’s Sub Area Activity Shimonoseki, the mine was discovered on July 3 during an underwater survey for the construction of an offshore wind power plant. The mine was about 2 meters long and about 60 centimeters in diameter. It is believed to have contained about 450 kilograms of gunpowder.

The mine is believed to have been dropped by a U.S. B-29 bomber sometime between March and August 1945 and was found on the seabed at a depth of 22 meters about 4 kilometers from shore. Thirteen bombs and artillery shells were also found scattered in the area.

For the detonation, vessels were prohibited to enter an area within a radius of 300 meters from the point where the mine was discovered.

The operation took about seven hours to carry out. MSDF personal took a boat from the minesweeper to the site where the mine was discovered. They attached an explosive to the mine and detonated it by remote control after leaving the site.

A mine also was blown up underwater in May 2014 off the coast of Sanyoonoda, Yamaguchi Prefecture, near the Kanmon Straits. “I realized that the effects of the war still remain today,” said a 62-year-old head of a fisheries cooperative’s branch office in Kitakyushu.

‘Starvation operation’

According to the MSDF’s Shimonoseki unit, the U.S. military is believed to have dropped about 12,000 mines around major ports and straits in Japan during the war. Nearly half of these, or about 5,000 mines, were scattered around the Kanmon Straits, including the location where the mine was disposed of on Sept. 25. The mine-laying was called “starvation operation.” Its purpose was to blockade the straits, a strategic point for maritime transportation, and to cut off the supply of goods.

Some estimates suggest that more than 1,000 mines remain in the area. There are various types of mines, including those that explode when they come into contact with a vessel, as well as those that detonate when they detect magnetic forces or the sound of propellers. Although they are highly unlikely to explode because of corrosion, they may blow up if a strong impact is applied.

“History shows that many mines were scattered in the sea area around the Kanmon Straits. Disposing of them is dangerous work, but we take care of it while ensuring safety,” said the MSDF lieutenant commander who led the Sept. 25 operation.

Advanced skills required for disposal

Regarding unexploded ordnance from the war, attention is often focused on artillery shells and bombs found on the ground. According to the Defense Ministry, 1,372 unexploded bombs and shells were disposed of last fiscal year. Zero to 12 mines have also been disposed of per year since fiscal 1975, when statistics first became available.

Under the San Francisco Peace Treaty, which was signed after the end of World War II, the disposal of unexploded ordnance is considered the responsibility of the Japanese government, and operations are handled by the Self-Defense Forces.

It is easy to lay mines, but disposing of them requires advanced skills and technology. Mines still remaining in the sea after the war can have serious consequences.

Civilians were killed in a mine explosion after the war. On March 30, 1949, a mine drifted ashore in Nadachi (present-day Joetsu), Niigata Prefecture, and exploded after it hit a rock. Sixty-three people in the area died. They included 13 infants, 36 elementary and seven junior high school students.

The MSDF traditionally clears mines in the sea and is recognized as a specialist of minesweeping.

In 1991, the MSDF disposed of 34 mines laid by Iraq in the Persian Gulf after the Gulf War. Although the work was done in murky waters with desert sand, the operation was completed without a single accident. This was the first overseas deployment of the SDF.

Mines are used in warfare today, too. After Russia launched its aggression against Ukraine, the Russian army is believed to have laid mines around Ukrainian ports on the Black Sea to obstruct the transport of grain.