Thermal neutrons are produced in the subsurface through (a) cosmic radiation interacting with material beneath the surface of the land, (b) alpha-capture reactions with light elements, and (c) natural fission of uranium. Field measurements of thermal neutrons in mines deeper than 800 m yielded counts which varied between 1.1 and 33 neutrons per cm3 per year. These neutrons originated primarily from alpha capture and fission reaction sources. Larger counts, up to 116 neutrons per cm3 per year, were obtained from the shallow subsurface where neutrons associated with cosmic ray interactions probably accounted for a significant fraction of the neutrons detected. With only one exception, more thermal neutrons were detected than were predicted on the basis of the uranium and thorium concentrations and the overall chemical composition of rocks surrounding the measuring stations.