A method and apparatus for detecting objects located underground. In one advantageous embodiment, a detection system detects objects having electrical non-linear characteristics located underground. The detection system comprises a transmitter unit, a receiver, and a processor. The transmitter transmits a plurality of pulsed radio frequency signals having a first frequency and a second frequency into a ground. The receiver monitors for a response radio frequency signal having a frequency equal to a difference between the first frequency and a second frequency, wherein the response radio frequency signal is generated by an object having the non-linear conductive characteristics in response to receiving the plurality of electromagnetic signals. The processor is connected to the transmitter unit and the receiver, wherein the processor controls an operation of the transmitter unit and the receiver, wherein the object is detected when the response radio frequency signal is detected by the receiver.