The prefecture of Gunma, north of Tokyo, had no history of frequent or severe earthquakes.

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For 84-year-old Saburo Ienaga, a former history professor at Tokyo University of Education, the ruling marked the end of his 32-year battle against state censorship.

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"Methane hydrate was a key cause of the global warming that led to one of the largest extinctions in the earth's history, " Ryo Matsumoto, a professor at the University of Tokyo who has spent 20 years researching the subject, told Bloomberg in December.

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