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Are methane hydrates the next big energy source? Japan hopes so.

Japan is in a tough spot, energy-wise. The nation imports nearly all of its oil and natural gas. Most of its nuclear reactors have been shut down after Fukushima. Wind and solar are still in the early stages of ramping up.

That explains why the country is looking for energy in unexpected places. Japan is currently trying to tap into undersea deposits of methane hydrates — also known as "fire ice" — in hopes of converting the trapped methane into usable natural gas.

On Tuesday, Japan announced a major new breakthrough. For the first time, a team aboard the drilling ship Chikyu had successfully extracted gas from a layer of methane hydrates 1,000 feet below the seabed in the Eastern Nankai Trough.

Now, this is just an initial step — albeit a big one. The Japanese government says that it's still five years away from commercial extraction of natural gas from methane hydrates. The drilling process itself is still tricky and expensive. But if Japan figures out how to unlock these hydrates, it could have huge implications for both energy and climate change.

Here's a quick primer on the issue:

What are methane hydrates? Methane hydrates are essentially cage-like lattices of water molecules that contain methane, the chief ingredient in natural gas. They can be found either beneath the seafloor or underneath Arctic permafrost:

Where are they found? All over the world. Right now, Japan is trying to extract methane from offshore gas hydrates in the Sea of Japan. The United States is currently funding 14 different research projects into the energy source after a successful test on Alaska's North Slope:

How much energy are we talking about? Potentially, a staggering amount. The U.S. Geological Survey estimates that gas hydrates could contain between 10,000 trillion cubic feet to more than 100,000 trillion cubic feet of natural gas.

Some of that gas will never be accessible at reasonable prices. But if even a fraction of that total can be commercially extracted, that's an enormous amount. To put this in context, U.S. shale reserves are estimated to contain 827 trillion cubic feet of natural gas.

Um, wouldn't burning all that fuel heat up the planet? Yes, if enough natural gas was extracted and burned, that would produce a whole lot of carbon-dioxide emissions. The U.S. Geological Survey estimates that there's more carbon trapped inside gas hydrates than is contained in all known reserves of fossil fuels:

Now, that might be a high-end estimate. More recent studies have suggested that the hydrates contain somewhere between 500 and 2,500 gigatons of carbon-dioxide. Even that lower figure, however, is more than twice as much as the carbon contained in natural gas from all of Earth's other sources.

Bottom line: It could prove impossible to keep global warming below the goal of 2°C if a significant fraction of this natural gas gets burned.

Why are methane hydrates so tricky to exploit? In part because most of the hydrates are located either in colder environments or deep underwater, places that are simply difficult and expensive to drill in. As a U.S. Geological Survey study (pdf) notes, "Technologies exist to address all of these issues, but their implementation will add to overall development costs for producing natural gas from hydrate."

The U.S. Energy Information Administration is even more skeptical: "So far, gas hydrates have provided more problems than solutions."

Meanwhile, drillers also have to be careful to not let the methane trapped in the hydrates leak out. When methane is released directly into the atmosphere, it's an even more potent greenhouse gas than carbon-dioxide. Already, climate scientists have raised concerns about methane from hydrates bubbling up naturally as the Earth warms and permafrost melts — which, in turn, heats the Earth even further. Drilling could exacerbate that problem without proper precautions.

Is there a cleaner way to harness methane hydrates? Possibly. Researchers have been wondering if it might be possible to pump carbon-dioxide back into the undersea lattices once the methane has been extracted — making the whole thing carbon neutral. A test at Conoco-Phillips' oil field in the North Slope of Alaska last year showed this was technically possible. So far, though, this idea is very far from a reality.

Does Japan have any other, cleaner options for energy? Potentially, yes. The legislature introduced new incentives for renewable energy last July, and already wind and solar installations are surging. Meanwhile, Japan has the third-largest geothermal resources in the world, with enough to satisfy nearly 10 percent of the nation's energy needs. There's also the possibility that Japan will restart many of its currently shuttered nuclear reactors.

So gas hydrates are still a ways off? Yes, but don't be surprised if we hear a lot more about them in the next few years.