The fires consume up to 200 million tons of coal a year, equivalent to between 10 and 20 per cent of the national coal production.

Ni Bin, an expert with the China National Administration of Coal Geology, said the underground fires are always started by lightning or spontaneous combustion in coal mines. Some have been smouldering for thousands of years and may trigger chains of environmental disasters, he said.

One of the biggest problems with these fires is that they are difficult to find.

The traditional methods of outdoor manual investigations and geophysical surveys provide only sporadic information on underground fires, Ni said.

John van Genderen, a geomorphologist at the International Institute for Aerospace Survey and Earth Sciences (ITC) in the Netherlands, applied remote-sensing technology to find coalfield fires. He has contributed to building up an international high-tech counterattack after many years of study and co-operation with his Chinese partners.

Heat measurements from satellites, planes and ground-based detectors have been combined to determine the size and depth of the fires and the direction in which they are burning.

Due to its unique geographical conditions and its climate, China is the country most severely hit by coalfield fires, which have caused annual economic losses of up to 4 billion yuan (US$480 million) and produced 1 million tons of noxious gases, such as carbon monoxide and hydrogen sulphide.

Coalfield fires in northern China alone are responsible for between 2 and 3 per cent of worldwide emissions of carbon dioxide, the main greenhouse-effect gas.

After over 10 years of geological surveys, Chinese experts have mapped out 56 coal mines burning over an area of more than 720 square kilometres in areas such as Gansu Province, the Ningxia Hui Autonomous Region, Qinghai Province, Shaanxi Province and the Xinjiang Uygur Autonomous Region in Northwest China and the Inner Mongolia Autonomous Region and Shanxi Province in North China.

Professor Van Genderen said: "Xinjiang alone contains more than 80 coalfields and each of these has between 20 and 30 individual fires burning in them, out of control."

(China Daily May 27, 2003)