When stimulated with a standard marker locator, the marker balls generate an exactly measurable high-frequency field for centimeter-accurate location at depths of up to 1.5 meters. Unique patented design for superior performance characteristics regardless of the position of the marker ball. Environmentally neutral due to the absence of potentially hazardous chemicals in the markers. The purely passive construction of the marker ensures its reliable function for decades throughout the entire lifespan of the underground installation.