Commercial development of the globe's huge reserves of a frozen fossil fuel known as "combustible ice" has moved closer to reality after Japan and China successfully extracted the material from the sea floor off their coastlines.
Combustible ice is a frozen mixture of water and concentrated natural gas. Technically known as methane hydrate, it can be lit on fire in its frozen state and is believed to be one of the world's most abundant fossil fuels.
But experts said large-scale production remains many years away, and if not done properly could flood the atmosphere with climate-changing greenhouse gases.
The official Chinese news agency Xinhua reported that the fuel was successfully mined by a drilling rig in the South China Sea on Thursday. Minister of land and resources Jiang Daming declared the event a breakthrough moment heralding a potential "global energy revolution".
A drilling crew in Japan reported a similar successful operation two weeks earlier, on May 4 off the Shima peninsula.
For Japan, methane hydrate offers the chance to reduce its heavy reliance on imported fuels if it can tap into reserves off its coastline. In China, it could serve as a cleaner substitute for coal-burning power plants and steel factories that have polluted much of the country with lung-damaging smog.
The South China Sea has become a focal point of regional political tensions as China has claimed huge areas of disputed territory. Previous sea oil exploration efforts by China met resistance, especially from Vietnam, but its methane hydrate operation was said to be outside the most hotly contested areas.
Methane hydrate has been found beneath sea floors and buried inside Arctic permafrost and under Antarctic ice. The US and India also have research programmes pursuing technologies to capture the fuel.
Estimates of worldwide reserves range from 280 trillion to 2,800 trillion cubic metres, according to the US Energy Information Administration. By comparison, worldwide production of natural gas was 3.5 billion cubic metres in 2015, the most recent year available.
That means methane hydrate reserves could meet global gas demands for 80 to 800 years at current consumption rates.
Efforts to extract the fuel at a profit have eluded private and state-owned energy companies for decades, in part because of the high cost of extraction techniques, which can use large amounts of water or carbon dioxide to flood methane hydrate reserves so the fuel can be released and brought to the surface.
Japan first extracted some of the material in 2013 but ended the effort due to sand from the sea floor clogging machinery, according to the Ministry of Economy, Trade and Tourism.
There are also environmental concerns.
If methane hydrate leaks during the extraction process, it can increase greenhouse gas emissions. The fuel also could displace renewables such as solar and wind power, said David Sandalow, a former senior official with the US State Department now at Columbia University's Centre on Global Energy Policy.
However, if it can be used without leaking, it has the potential to replace dirtier coal in the power sector.
"The climate implications of producing natural gas hydrates are complicated. There are potential benefits, but substantial risks," Mr Sandalow said.
Commercial-scale production could be "transformative for north-east Asia, particularly for Japan, which imports nearly all its hydrocarbon needs," said James Taverner, a senior energy industry researcher at IHS Market, a London-based consulting firm.
The consensus in the industry is that commercial development will not happen until at least 2030. Smaller scale output could happen as early as 2020, said Tim Collett, a scientist with the US Geological Survey.