A model has been developed to explore the late-time evolution of temperature in the rubble -filled chimney that is formed following the collapse of the cavity produced by an underground nuclear detonation. It is assumed that thermal convection currents sustained by energy from a hot solidified melt at the bottom of the chimney circulate sufficiently rapidly that within a few weeks after the explosion they are able to maintain, the chimney as an isothermal region. On the time scales of interest (months) the temperature of this region is governed by heat conduction into the initially cold rock surrounding the chimney and melt. The model, when applied to the Gasbuggy and Rulison chimneys, is capable of predicting temperatures which compare favorably with experiment, and allows rapid exploration of sensitivity of chimney temperatures to variations in physical and geometric parameters. The sensitivity of calculated temperatures in Gasbuggy to uncertainties in geometrical factors (radius, etc.), the spatial partition of thermal energy produced by the blast, and the physical properties of the rock (density, specific heat, thermal conductivity) is determined. Finally, having calibrated the model against Gasbuggy and Rulison experiments, it is estimated that temperature increases in the anticipated chimney formed by the proposed Wagon Wheel experiment will be in the range 725 to 550°F in the 1- to 24-mo period following the event. These temperatures are much higher than those in Gasbuggy and Rulison, and raise questions of the occurrence of CO2-producing reactions throughout the Wagon Wheel chimney volume. The implications of these high temperatures for gas production equipment should be investigated.