Hiroshima’s Okunoshima Island: WWII poison gas factory uncovered

Okunoshima Island, once a secret poison gas production site, serves as a reminder of its dark history. Photo / The Japan News via The Washington Post

During World War II, an entire island in the Seto Inland Sea vanished from the map. It didn’t sink into the sea – rather, Okunoshima Island in Hiroshima Prefecture was kept shrouded in secrecy because it was home to a facility where the Imperial Japanese Army was producing poison gases. Even now, the ruins that remain on this small island, just 4km around, continue to serve as a reminder of its dark history.

Six tanks with a capacity of about 100 tonnes each still sit in an old poison gas storehouse. The building’s exterior walls are blackened in places where they were burned by flamethrowers after the war to remove all remnants of poison. Another building houses a diesel generator that used to supply electricity to a poison gas factory. Its interior, with a ceiling three storeys high, overwhelms visitors.

Okunoshima Island is located about 50km east of the centre of Hiroshima city. A 15-minute ferry ride brings visitors from the nearby harbour to its shores. In late April, 80-year-old Masayuki Yamauchi, who teaches people about the factory’s history, met a group of junior high school students who came for a field trip.

“The Japanese were not only victims of the war but also aggressors. I want you to know that,” he told the students.

The poison gas factory opened as a weapons plant for the army in 1929. The island was ideal not only because it was easy to keep secret, but also because workers could easily come and go by boat.

Okunoshima Island housed a poison gas factory during World War II. Photo / The Japan News via The Washington Post

The factory produced mustard gas and lewisite, both deadly poisons that cause severe skin burns and blisters. After the war against China started in 1937, production of the gases increased at the factory. Some of the gases are believed to have been used on the battlefield. Records show the factory produced 6616 tons of toxic gases through to the end of the war and more than 6000 people were involved in producing them.

Residents from the mainland were mobilised to work at the factory, including students and even girls. Danger was a constant companion to the workers, and one after another they accidentally breathed in toxic gas and became injured or died. They were ordered not to say anything about the factory, and the island and the surrounding area were covered with a white blur on maps.

After the war, the unused gases were disposed of by dumping them in the sea or incinerating them. But many former workers continued to suffer from chronic bronchitis or other illnesses. According to Hiroshima Prefecture, as of the end of May, there were still 463 living people, with an average age of 95, who had been certified by the Government as suffering from health issues as a result of working at the factory.

“In Hiroshima Prefecture, damage caused by the US military tends to get most of the attention, but now I’ve learned that Japan was an aggressor, too,” said a 14-year-old student from the prefecture. “It made me think that there’s no war in which only one side is bad,” she added.

‘So the evil won’t be repeated’

Yamauchi was born in 1944 in Manchuria in what is now northeastern China. While he was growing up, his mother would often tell him that they had been able to return to Japan thanks to the support of Chinese people.

Yamauchi has lived in Takehara, Hiroshima Prefecture, since the end of the war. In 1996, when he was working as a social studies teacher at a high school, he participated in a local symposium about the poison gas factory.

At the symposium, he learned that poison gases the Imperial Japanese Army dumped in China caused harm to local people. Discovering this scar of the war shocked him immensely. He then joined a citizens’ group and began to engage in history-telling activities on Okunoshima Island.

However, he feels that field trips to the island have been decreasing in recent years.

In fiscal 2024, a record 2.26 million people visited the Hiroshima Peace Memorial Museum in Hiroshima, which chronicles the atomic bombing tragedy. But the Okunoshima Poison Gas Museum received only 42,000 visitors, even though both museums are in the same prefecture. The number of people coming to the Okunoshima museum has yet to return to its pre-pandemic level.

The island, known for rewilded rabbits, receives increasingly fewer visits to its poison gas museum. PHoto / Chris McGrath, Getty Images

The island is, however, growing in popularity as a “rabbit island” where visitors can meet rewilded rabbits.

Yamauchi has vowed not to allow the island’s past with the poison gas factory to fade away.

“The damage done by the atomic bomb is known all over the world, but hardly anyone knows about the poison gas on Okunoshima,” Yamauchi said. “I’ll keep on telling people the history [of the island] so that the same evil won’t be repeated.”

Dark tourism growing popular

Trips to war-related historical sites and ruins are examples of “dark tourism”, which began to be advocated for in Britain in the 1990s. Famous destinations include the former Auschwitz concentration camp in Poland where Jewish people were massacred by Nazi Germany. An example in Japan is the atomic bomb dome in Hiroshima.

In Tokyo, there is the former Hitachi aircraft Tachikawa factory electrical substation in the city of Higashi-Yamato. The facility used to supply electricity to a factory that manufactured engines for military planes. The building was strafed three times between February and April 1945, and there are still many bullet holes in the exterior walls. This building was spared the fate of demolition thanks to a campaign by residents in the area.

“The bullet holes are concentrated on the south side of the building, which tells the flight route of the US planes and lets us experience the horror of the time,” said Hirotoshi Kosuda, 78, the head of a preservation group for the facility. “There are quite a few visitors in their 20s and 30s, too, who don’t know about the war.”