The Wuda Coalfield, Inner Mongolia, China, is one of the largest coalfields suffering from coal fires in China, and has experienced fires for > 50 years. With the aim of observing the active state of underground coal seam fires to protect the main roadways of the coal mine, 44 drill holes were drilled in five fire areas of the Suhaitu coal mine to monitor gas emissions. A LumexRA-915 + Portable Mercury Analyzer was used to measure mercury concentrations in the exhaust gas. The drill holes exhibited high gaseous mercury concentrations, with an average of 4165 ng/m(3) (34-62,513 ng/m(3), n = 1584), which was 45 times higher than that measured in the near-surface atmosphere around the drill holes. This finding indicates that the mercury emissions originate from underground coal fires. Furthermore, mercury concentration has a positive correlation with CO content and gas temperature, implying that mercury has the potential to act as a supplementary coal-fire index gases to monitor the prevailing underground coal fire in north China on the basis of traditional indicators. Whether it can perform satisfactorily in practical applications requires further comprehensive study.