During World War II, Japan experimented with chemical weapons. Ōkunoshima Island was used by the Imperial Japanese Army as a secret manufacturing site. The island’s secluded location made it an ideal place to produce and test such weapons, including mustard gas and tear gas. The facility operated under a veil of secrecy – the surrounding waters were heavily patrolled to prevent information leaks.

At the end of World War II, the facility was destroyed, and the remaining chemicals were supposed to be cleaned up. As it turned out, many of the weapons and containers were hastily dumped into the ocean, leading to environmental concerns in the years that followed. During its wartime operation, rabbits were brought to the facility’s test site as subjects for chemical weapons. When the factory was abandoned, the rabbits were either released or escaped into the wild. In the absence of any human interference and the relatively low predator population (such as foxes or large birds), the rabbits were able to reproduce and thrive. Eventually, thousands of rabbits were discovered to have established a particular ecosystem on the island.

Over the years, Ōkunoshima Island gained popularity as a tourist destination. People visited the island, bringing food and, in this way, sustaining the rabbit population. At the present time, it is estimated that 900 to 1100 rabbits live on Ōkunoshima Island.