British actress Millie Bobby Brown and Danish actor Mads Mikkelsen break the sake barrel during the opening ceremony for Osaka Comic Con 2023 at the Intex Osaka on May 5, 2023 in Osaka, Japan. (Photo by Jun Sato/WireImage)

A firefighter works outside of a building where an explosion and a fire broke out in Shimbashi area of Tokyo on July 3, 2023. (Photo by Yuichi Yamazaki/AFP Photo)

Japanese professional wrestler Hiromu Takahashi attends a promotional event of a dinosaur show “Dino Safari Gigant” in Tokyo on Wednesday, August 2, 2023. A dinosaur show using dinosaur shaped mrchanical suits will be performed in Yokohama from August 10. (Photo by Yoshio Tsunoda/AFLO/Rex Features/Shutterstock)

Minoru Suzuki and Sanshiro Takagi, wrestlers from Japanese professional wrestling promotion DDT Pro-Wrestling, fight inside Tokaido Shinkansen bullet train from Tokyo to Nagoya during a wrestling show in Japan on September 18, 2023, in this handout photo released by JR Tokai (The Central Japan Railway Company). (Photo by JR Tokai/Handout via Reuters)

Men ride on a “mikoshi” or portable shrine as local people carry it into the sea during a festival to wish peace in the ocean and good fortune in the new year in Oiso, west of Tokyo, January 1, 2015. (Photo by Yuya Shino/Reuters)

A rickshaw man runs under the blooming cherry blossoms in Tokyo, Sunday, March 29, 2015. (Photo by Shizuo Kambayashi/AP Photo)

An employee of Daikin Industries Ltd works the production line of outdoor air conditioning units at the company's Kusatsu factory in Shiga prefecture, western Japan March 20, 2015. As Japan heads into the season of peak demand for room air-conditioners, Daikin managers have been tasked with figuring out how to boost output by some 20 percent at the 45-year-old Kusatsu plant that six years ago the company had almost given up on as unprofitable. (Photo by Yuya Shino/Reuters)

Only after World War II did the secret spill: Ōkunoshima, located in the Inland Sea of Japan between Hiroshima and Shikoku, was the top-secret site for manufacturing chemical warfare. When the factories were closed down, a number of exotic wild rabbits were seen freely roaming the island. They were assumed to have been the test subjects for the chemical weapons, which the military failed to eradicate when the factory was demolished.