While the earth provides a relative secure environment in the event of flood, fires and storm events, the design of antennas which operate satisfactorily to a receiver on or above the ground has been a design challenge. A cavity-backed slot antenna previously used for biomedical applications was modified for subsurface propagation at 433 MHz at shallow depths in soil. A 433 MHz +10 dBm beacon located in an aluminium box (163 × 114 × 42 mm) with a thin (1 mm width) slot buried to a depth of 100 mm. Vertical electric field measurements were made on the surface of the earth from the antenna buried in moist lawn-covered soil. The antenna was placed below the grassed surface with the slot directing radiation horizontally into the soil. Propagation across the surface suffered a loss of 0.53 dB/m. This technology shows significant promise in the case of sensors buried in soil.