The Lotts Creek coal mine fire developed in an abandoned underground and surface mine in the Hindman (Hazard No. 9) coalbed in the Hazard North 7 1/2 -minute quadrangle. In a study conducted in April and May, 2012, an array of five thermocouples, with 1-m vertical separation between individual thermocouples, was suspended in an active vent of the fire. Data were collected from each thermocouple for >21 days at 1-min intervals. For many days, small thermal gradients were noted. On 3 May 2012, the 10th day of the investigation, a relatively extreme thermal event occurred, with a nearly 95 degrees C difference in the 2 m between the 2nd and 4th thermocouples. Other extreme heating events were noted, but none as intense as the latter event. Such events were overprinted on the diurnal, and shorter, breathing cycles of the fire and the complications induced by changing atmospheric conditions and the shifting of the fire front as the coal burned. The 3 May thermal event may have been a result of a sudden opening of a vent, with the consequent influx of air increasing the intensity of the fire. Since the experiment was run remotely without visits to the site between the time of placement and pick-up of the data logger assembly, no one witnessed the incident. (C) 2012 Elsevier B.V. All rights reserved.