Underground Coal Gasification (UCG) is a process for exploiting coal that cannot be mined because the seams are too deep, thin or fractured. The process involves using the same sort of drilling technology usually used for fracking to get air/oxygen into the coal seam and then set the seam on fire.
By controlling the amount of oxygen injected it is then possible to only partially burn the coal and bring the gases produced to the surface where they can be burn to produce energy.
A witches brew of toxic and carcinogenic coal tars are produced in the burn cavity. The process is associated with serious groundwater contamination and massive carbon emissions.
Small scale tests of UCG have been taking place on and off since the 1930’s, particularly in the Soviet Union and United States, and have usually resulted in contamination of groundwater. More recently there have been three tests in Australia, two of which have resulted in the plants being shutdown.
After only a five day burn the well at the Cougar Energy plant in Kingaroy, Queensland exploded and subsequently benzene and toluene were detected in groundwater and in the fat of animal grazing on the surface.
Full scale UCG would likely involve huge plants connected to multiple gasifiers, and might be similar to tar sands extraction in its scale and impact.
See Underground Coal Gasification: Hellfire and Damnation for more details.