Published online 22 November 2007 | Nature | doi:10.1038/news.2007.281

News: Briefing

An underground coal fire has been snuffed out in China after decades of burning. Nature News finds out what happened.

This Pennsylvanian coal-seam fire periodically burns to the surface, emitting smoke and noxious fumes. US Office of Surface Mining

How can there be a fire underground?

This fire was in the Terak coalfield of Urumqi, in the Xinjiang region of China. It was the coal that was on fire — and had been for at least 50 years.

Such fires sound bizarre, but they aren't actually too unusual. Similar underground fires hit coal regions around the world: there’s a lot of fuel down there to burn.

The mining town of Centralia, in Pennsylvania, was almost entirely evacuated in 1984 because of a coal fire. The fire started in 1962, when a landfill site was set alight by town officials. The landfill was in an old mine shaft, which subsequently burned and spread under the town.

There is also fossil evidence that coal fires raged in prehistoric times.

50 years seems like a long time to burn.

It is. But there are fires that have been going much longer than that. India has lots of them, some of which have been burning since 1916. In 2004, another fire near Urumqi was finally extinguished after burning for 130 years. Australia has the Burning Mountain, near Wingen in New South Wales, which is a coal seam that has been burning for an estimated 6,000 years.

How does a fire like this get started?

To start, a fire needs heat, fuel and oxygen. A coal seam provides an abundant fuel source. It seems unlikely that coal deep underground would get any oxygen, but mining changes that. “As soon as mining takes place, then oxygen can circulate,” says Paul Van Dijk at the International Institute for Geo-Information Science and Earth Observation, Enschede, the Netherlands. Heat can be provided by sunlight, or, under certain conditions, by chemical reactions. Other times it's simply started by a match. Most mine-related fires are started by humans, says Claudia Künzer at the Vienna University of Technology, Austria. A miner might drop a cigarette or cook on an open fire.

How did they put this fire out?

The fire covered some 923,500 square metres, fuelled by coal 100 metres below ground. Such extensive underground fires are not easy to tame, and attempts to do so are often unsuccessful, says Van Dijk. The report from the Chinese Coalfield Fire Fighting Project Office says that they drilled into the ground and poured down slurry and water to remove heat, then covered the surface to cut off the oxygen supply. Even then, the area will be closely monitored until 2009 to check that it doesn’t reignite. The tiniest whiff of oxygen could set the whole thing going again.

“It’s possible to put them out, but it’s difficult,” says Künzer.

Are there other ways of taming these underground beasts?

Other methods that have been tried include liquid nitrogen, which provides a double whammy by being both very cold and smothering the supply of oxygen. Synthetic materials, such as foams, are also used. Pouring on a blanket of mud can stop a fire, but because the coal is still hot, the mud can dry and cracks can form, allowing oxygen to seep back into the system.

How dangerous are these fires?

They can make the ground unstable, so that holes suddenly appear underfoot. But perhaps the main problem is economic: allowing the coal to burn uses up a local source of income.

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And what about the environment?