Coal fire sponge (CFS) is a type of sponge-like contaminated soil found at surface vents of coal fire commonly seen in global coal fire regions. The sponge and the related soil samples were collected in Wuda coalfield, Inner Mongolia of China, and measured by Lumex RA-915+ Mercury Analyzer with PYRO-915 pyrolysis cell. The results show that the mercury (Hg) concentration in CFS reaches 13,967 ng/g (2653-38,470 ng/g, n = 73), about 1400 times of local soil background level (10 ng/g), which has practically become a point source of heavy Hg pollution. The 8000 m(2) reclamation zone that CFS resides also shows elevated Hg level of 2536 ng/g (1438-4313 ng/g, n = 20 x 5), which is the result of weathering and dissemination of CFS, and also has become an area source of Hg pollution. Both findings suggest that the formation of CFS on coalfield ground surface has changed the environmental fate of Hg from entering atmosphere to staying locally, causing serious surface atmosphere, soil, and water body contamination in the coalfield. In addition, CFS also shows potential as a sensible indicator of underground coal fire remainder in reclamation/coal fire treatment areas.