1. A method for detecting tunnels within an area comprising the steps of: providing horizontal tensor gravity gradients for the area; calculating a plurality of gravity gradient vector field pairs from the horizontal tensor gravity gradients, each gravity gradient vector field pair including one gravity gradient vector field and a respective orthogonal gravity gradient vector field; detecting dipolar points and polar source points in each of said one and respective orthogonal gravity gradient vector fields for each gravity gradient vector field pair; identifying said polar source points that are present in one but not in both of said one gravity gradient vector field and said respective orthogonal gravity gradient vector field for each gravity gradient vector field pair to identify endpoints; and distinguishing said endpoints resulting from surface features from said endpoints resulting from tunnels.