Geologic structures have been responsible for numerous underground accidents and fatalities, and are a constant source of downtime. During longwall mining, ground control problems associated with geologic anomalies are aggravated by the abutment pressures that develop as the panel is retreated. Therefore, it is imperative that these structures be recognized and properly supported in head and tail gates. Fractures, faults, clay veins, igneous dikes, and ancient stream channels (paleochannels) disrupt the lateral continuity of the mine roof and can cause falls in gateroads. Horsebacks and kettlebottoms are also hazardous because abutment pressures tend to "pop out" these structures into the gateroads. This U.S. Bureau of Mines investigation describes the physical characteristics associated with hazardous geologic structures, identifies the coalfields where they have been observed, and, where possible, suggests support strategies and other mine plan modifications to help minimize their effect.