Okunoshima was a tiny island used only by a few fishing families until the war between Japan and Russia. From 1927 to 1929 it was developed in secret, with the island's fish preservation processor being converted into a toxic gas reactor.
In 1925, Japan initiated production of chemical weapons in response to development in Europe and the U.S. Although Japan was a signatory to the 1925 Geneva Protocol banning use of chemical weapons, their production was not covered under the treaty. Still, secrecy was so important that the island was erased from some maps.
Over 6 kilotons of mustard gas and tear gas were produced on the island. Although areas accessible to the public are now completely safe, parts of the Poison Gas Factory are still sealed due to contamination.
Workers at the factory were not told what they were producing and subject to severe working conditions. Several decades after the end of World War Two, victims from the plant were given government aid for treatment and the Okunoshima Poison Gas Museum was opened in 1988.