Using three radio frequency electromagnetic field procedures: surface-to-surface, borehole-to-surface, and borehole-to-borehole, we detected and delineated a horizontal, 21/2 X 5-ft (3/4 X11/2-m), and 45-ft (14-m) deep tunnel crossing the US-Mexico border near Otay Mesa east of San Diego, CA. We used continuous monochromatic signals to delineate the tunnel by (1) locating and identifying signature variations in signal transmissivity from the tunnel void in the local rock (sandstone) and (2) inducing and mapping the (secondary) radiation from electrical cabling within and running the length of the tunnel. In the surface-to-surface study we evaluated both a single magnetic dipole and a dipole gradiometer receiver and both a magnetic dipole and a grounded long-wire transmitter operating between 22 kHz to approximately 300 kHz to isolate the induced radiation. We found that both sources operating at approximately 100 kHz gave the best transmitter-cable coupling, and the nulling effect of the gradiometer was necessary to eliminate the primary field. In the surface-to-borehole study we placed a dipole receiver in boreholes and a dipole transmitter on the surface and operated at approximately 100 kHz. Horizontal traverses of the transmitter found peaks in the signal amplitude when the transmitter was directly above the tunnel. Vertical traverses of the receiver found (secondary) radiation peaks in the signal amplitude at depths corresponding to the tunnel depth. In the borehole-to-borehole studies, using zero-offset (i.e., transmitter and receiver at the same depth) traverses and 1 MHz and 15 MHz signals, we found peaks in the signal amplitude at depths corresponding to direct ray transmission through the tunnel.