[Dedevil]

This is a small magnetic abnornmallity of the substance in the ground. Normally we get a compass and see with our eyes North. This is a global whole of the earth occurance because most substances that are solid in the ground and not moving are mostly pointing north. But some sustances are Not in the ground and pointing north so you get a slight magnetic field abnormality. If you take a sensitive compass there i think the North will change direction as you go over the substance you have found. Think of it like a magnetised wire on a string. It will point north Yes? But if you move the wire with your finger (a stronger force) the north direction has now changed.
rgds.