A memorial ceremony was held on 9 November 2013 to mark the 50th anniversary of the country’s worst mine accident in the postwar era.  An explosion in 1963 at the Mitsui Miike coal mine in Omuta, Fukuoka Prefecture, and Arao, Kumamoto Prefecture, which used to be one of the country’s largest, killed 458 workers. Another 839 suffered carbon monoxide poisoning. The mine, which closed down in 1997, was operated by Mitsui Mining Co. On November 9, 1963, the explosion occurred in a mine tunnel roughly 500 meters below the mine ground-level entrance. The blast and flame caused roof fall in many areas in the tunnels, which then quickly filled with carbon monoxide. It was the worst postwar mine disaster. Lack of safety provisions to prevent coal dust was the main cause. Coal dust was the cause. Coal-dust explosions are the worst type of explosion because there is a great amount of carbon monoxide produced. As a result of this, many mine workers continue to suffer from the long-term after-effects of carbon monoxide poisoning even if they are lucky enough to be rescued alive. Methane gas explosions create carbon monoxide when the density of the gas is high, but if there is not much gas it is more often than not dispersed in the air. However, in coal-dust explosions the story is a very different one. Coal dust, being a solid rather than a gas, does not burn completely, and high-density coal-dust clouds can be formed. This prevents adequate air circulation, contributing to the production of carbon monoxide. Even if a coal-dust explosion does not spread throughout the length and breadth of the mine, the resulting carbon monoxide gas does in fact spread in this way and all the workers are poisoned. The Miike mine explosion of 1963 was an example of this.  It was common knowledge that maintaining the site clean and making the area moist by watering could prevent such explosions. These easy preventive measures had been in place on a routine basis. However, after a strong labour dispute in 1960, the insufficient number of safety personnel, caused by the employer's productivity-first policy, failed to take those measures. In 1955 coal as a primary energy source provided 50 per cent of energy needs in Japan, but after the Miike labour dispute and mine explosion, the level decreased, dropping to 16.4 per cent by 1975, while the use of oil increased from 20.2 to 73.3 per cent. . The accident was second worst in Japanese mining history and one of the worst ever recorded in the world, and resulted in a series of lawsuits that dragged on until 1987, when it is reported that each of the victims' families was awarded the modest sum of US$1,800.