Sir

Your Nature News story '50-year-old fire put out' (doi:10.1038/news.2007.281) mentions that an underground coal fire in Xinjiang's Terak coalfield in China has been extinguished, as has another, near Urumqi, that had burnt for 130 years. However, we remain unconvinced that the Terak coalfield fire will not reignite.

As a result of more than three years' efforts by the Xinjiang Coalfield Fire-fighting Project Office, the coalfield fire in Liuhuanggou, near Urumqi, was put out in 2004. We have been detecting signs of revival since 2005, however, including sporadic heat anomalies in sub-zones and smoke escaping from surface fissures. Local blocking of the fires has caused a redistribution of ground stress, resulting in new fissures connecting with the surface. These fissures allow entry of oxygen and promote heat circulation. Meanwhile, mining in the vicinity may have disrupted fire-suppression measures, causing new fire spots to start up around the previous fire zone.

Since 2005, the Liuhuanggou coalfield roads have been levelled to facilitate fire-fighting operations. Coal that was previously hard to extract can now be easily mined, illegally, from the coal-seam outcrops just by stripping the soil overburden that was once covered for the purpose of firefighting.

Illegal mining may also have helped to revive the fire. In coalfields in Xiaolongkou and Xiaohuangshan, for example, where fires had been confirmed as extinguished in 2001 by the Chinese government, illegal mining activities have already caused the fires to revive.

The Chinese government should prohibit uncontrolled mining activities and maintain long-term monitoring in and around the extinguished fire zones to prevent the fires from reigniting.