Established geophysical methods for detecting subsurface cavities, such as Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT), are often compromised by interference from near-surface soil conditions. This paper introduces the application of a Superconducting Quantum Interference Device (SQUID) second-order gradiometer as a robust alternative that overcomes these limitations. By measuring the spatial gradient of the magnetic field, this technique inherently suppresses uniform background noise while enhancing signals from local anomalies. Experimental trials were conducted over normal ground and a pre-defined void, both uncovered and concealed by a soil layer. The results demonstrate that the SQUID gradiometer produces a distinct and repeatable anomaly signature at the edges of the cavity. Crucially, this detection signature remains consistent whether the soil layer is present or absent, proving the method’s immunity to interference from the overburden. This study validates SQUID second-order gradiometry as a highly reliable and specific method for detecting subsurface cavities, unaffected by the geological and environmental factors that limit conventional techniques.