Wireless Sensor Networks (WSN) can be useful in subsurface geo-event monitoring. Hazard monitoring and effective in-situ characterization are some key issues that can be addressed by use of WSN in the subsurface. In this study a new concept of WSN application for subsurface geo-event monitoring was investigated. The new concept of functional signal takes advantage of the variations of signal strength of radios transmitted between the distributed sensor nodes of a WSN to monitor and characterize the sensed area. Experiments demonstrated that calibrated wireless signal strength variations can be used as indicators to sense changes in the subsurface. Based on the experimental results the new approach can be exploited further to provide a sensing methodology to identify and monitor subsurface hazards through the evolution of select physical conditions in the subsurface. These subsurface hazards can range from intrusion of a chemical plume to the onset of a slide which could change markedly the physical properties of the host soil, hence the functional signal transmitted with an embedded WSN.