The last decade has seen a growing acceptance of the magnetometric resistivity (MMR) method as a viable exploration technique in various geologic environments. Until recently, MMR exploration was carried out with both current electrodes and recording magnetometer located on the surface of the Earth. Significant improvements in anomaly amplitude can be achieved by lowering the recording magnetometer inside a drill hole. In contrast, the lowering of current electrodes beneath the surface does not always improve surface MMR responses. The advantages of locating the magnetic detector in a drill hole are illustrated numerically, anomaly calculations being carried out with a novel yet simple integral equation technique for a plate-like body. The practicality of the cross-hole MMR technique is demonstrated with a successful case history. Massive sulfide mineralization is mapped at a depth exceeding 500 m.