This story map is the digital version of the IGWS publication Special Report 74. Underground coal gasification (UCG) is a technology that can complement traditional methods of coal mining and surface gasification. This technology works by converting coal into a combustible gas (syngas) that can be used for industrial heating, power generation, and the manufacture of hydrogen, synthetic natural gas, or diesel fuel. In the UCG process, the cavity formed in the coal seam becomes the reactor, so the gasification of coal takes place underground instead of in a gasifier at the surface, as in the surface gasification process. Selecting appropriate sites for UCG plants and applying the best UCG technologies and practices are complex processes, and a variety of technical and geological factors must be taken into consideration to evaluate each potential site.