Studies on the growth of three-dimensional cavity geometries in underground coal gasification (UCG) are important in exploiting the large fraction of coal that is present in underground coal seams. In the present study, the cavity formation in UCG has been simulated using experiments carried out in three configurations: (i) sublimation experiments in camphor simulating primarily the heat transfer aspects, (ii) bore hole combustion in Acacia nilotica wood bringing in chemical reaction into play, and (iii) bore hole combustion a coal block bringing into consideration the effect of ash on the cavity formation. In all the three cases, the time-evolution of the cavity shape has been monitored under constant oxygen flow rate conditions by measuring the cavity shape and size at periodic intervals. Results show that the cavity formation rates as well as the shape of the cavity are significantly affected by the oxidant flow rate. The importance of the ash present in the coal on the cavity growth has also been brought out. A fair amount of gasification leading to the formation of H2, CO and CH4 was observed; this is shown to depend both on the inherent moisture as well as on the reaction zone temperature. © 2011 Elsevier Ltd.