A terrestrial magnetism responsive device adapted to use 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 being 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. The method includes the releasing of a north or south seeking magnet in the housing so that it is free to move easterly or westerly on the spring, moving the housing over the areas to be explored, and recording the east/west diversions of the magnet and the exact locations of the diversions.