The compass app on your phone probably won’t work because it relies on radio signals that are easily blocked by rock or water, but for a compass with a wobbly needle, it just depends on how far underground. A compass works because its magnetised needle lines up with the magnetic fi eld that runs between Earth’s north and south poles, and that fi eld is just as powerful if you go down a mineshaft or into the depths of the ocean. But the fi eld is created by swirling molten iron in Earth’s core, and if you could drill that far down, you’d fi nd your magnetic needle going haywire.
