Ground Penetrating Radar (GPR) identifies subsurface features by distinguishing materials with different dielectric constants and electrical conductivities. Subsurface cavities can, therefore, be detected by the variation in their electrical properties from the electrical properties of the surrounding material. To test the cavity detection ability of GPR, subsurface cavities of varying size, shape and content were modeled. Radar response to the cavity models was found to be affected by the composition of the surrounding soil material, the depth of the groundwater table, and the radar signal frequency. Based on knowledge gained from the cavity modeling study, a natural subsurface cavity was identified during a GPR field investigation. Limestone features such as bedding planes and fractures were mapped, and a detailed lake bottom profile was obtained by the radar system.