A terrestrial magnetism responsive device adapted to be worked using a north or south seeking magnet in both north and south headings, the device including a housing having a north or south seeking magnet therein. The magnet is suspended adjacent one pole by a sensitive spring to permit the magnet to be movable horizontally, when released, from a north pointing position or a south pointing position. Secured and balanced on the seeking or free end of the magnet is a soft iron rod, generally at ninety degrees to the magnet and having substantially the same diameter as the magnet. The rod is balanced along with the magnet so that the magnet and rod together will move horizontally. The housing is rotatable on a plate by which it is held and during operation is fixed on the plate to be moved horizontally in the direction of an area to be explored.The method includes the releasing of a north or south seeking magnet in the housing so that it is free to move east and west on the spring as the housing in its fixed directional position is moved over the areas to be explored. As it is moved, the east/west diversions of the magnet and the exact locations of the diversions are recorded to be used in determining the location of surface and subsurface local structures which have local magnetic force fields causing the diversions of the magnet.