The feasibility of producing a practical buried fiber optic sensor with high sensitivity for detecting intruders is demonstrated. Experiments were carried out with the use of an all fiber Michelson interferometer, the sensing arm of which was buried in sand. When the sensing arm was buried 8 inches deep in sand, the pressure length product required for a half fringe shift in: the sensor output was $1.09;kPa{cdot}m$. The relation between the applied weight and the phase change was almost linear. Experimental results indicated that the sensitivity of the optical fiber sensor was sufficient to detect people on foot and vehicles passing over the buried fiber.