1. A method of detecting subsurface tunnels, the method comprising: measuring air pressure continually in time with a first sensor positioned above ground to generate a first signal; measuring air pressure continually in time with a second sensor positioned at a depth relative to a surface of the earth to generate a second signal; generating a third signal using a numerical model that employs the first signal as input to produce an optimal approximation of the second signal, the accuracy of such approximation being optimized by systematic adjustment of model input parameters that represent the effects of the permeability of the earth and the depth of the second sensor on the difference of the second signal from the first signal; and detecting the introduction of a subsurface tunnel proximal to the second sensor based on a change in the accuracy with which the third signal approximates the second signal over subsequent time windows.