Studies of the remanence and rock magnetic properties of host-rock Early Tertiary Hanna Formation at the Hanna II, Phases 2 and 3 underground coal gasification (UCG) experiment, Hanna, Wyoming, have determined the nature of the remanent magnetism of unaltered Hanna Formation and changes in the remanence and magnetic mineralogy attending the UCG tests. Core holes ZHP-174, located in the center of their burn cavity, contain intervals approximately 10 m thick immediately above the burn cavity in which the natural remnant magnetization (NRM) intensity (greater than 1.0 A/m), NRM dispersion (k values exceeding 1000), NRM mean inclination (very close to 69, the ambient field inclination during burning), initial susceptibility (greater than 1.0 x 10 4), and demagnetization properties (distributed blocking temperatures and moderate coercivity spectra, suggesting significant coarse magnetite) all are anomalous in comparison to the "background" properties of fresh Hanna Formation. Post burn core holes located neat the edge of the burn cavity do not contain such intervals of anomalous magnetization properties. As well, Hanna Formation beneath the burn cavity is not remagnetized. The origin of the remagnetized zone is due to the production of I-5 volume percent secondary magnetite from detrital ferromagnetism silicates and possibly hematite. Thermal demagnetization experiments on fresh Hanna Formation show that magnetite production occurs in air at temperatures above about 300 C. The magnetic data have significant application toward better understanding UCG experiments and attending thermal anomalies. Thermal perturbations on surrounding Hanna Formation units must have been funneled vertically upward form the center of the burn cavity, not radically away from it. Assuming that magnetite production is solely temperature dependent, the thermal gradient above the burn cavity is estimated to be approximately 700 /m.