One cubic foot of solid methane hydrate yields about 164 cubic feet of gas.

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Methane has previously been extracted from methane hydrate buried deep under Arctic permafrost, but not from ocean deposits.

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Government officials have said that they aim to establish methane hydrate production technologies for practical use within five years.

BBC: Production test

They are also engaged in ongoing research into the amount of methane hydrate under this area of the ocean floor.

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Engineers used a depressurisation method that turns methane hydrate into methane gas.

BBC: Production test

"It is the world's first offshore experiment producing gas from methane hydrate, " an official from the economy, trade and industry ministry told the AFP news agency.

BBC: Production test

More than 50 million years ago, undersea landslides resulted in the release of methane gas from methane hydrate, which contributed to global warming that lasted tens of thousands of years.

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Other countries, including the US and Canada, are also conducting research on methane hydrate, which experts say is at least twice as plentiful as all known reserves of natural gas.

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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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Methane hydrates are frozen water molecules that trap methane gas molecules in a crystalline, lattice-like structure known as a hydrate.

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