Powered thermoluminescent dosimeters, CaSO4:Dy, were used to monitor underground radiation fields. Methods of monitoring at selected sites in Taiwan are described; exposure rates above ground at the monitoring sites are also included for comparison. Differences in exposure rates between above ground at 1 m and underground at -2 m may reach 0.516 nC kg-1 h-1. The key factor affecting exposure rates underground is the water content in soil or precipitation that may reduce the diffusion of Rn. The difference in exposure rates between rainy days and sunny days may reach 0.774 nC kg-1 h-1. An example of applying this monitoring technique at a radwaste repository is given.