An Echo in the Bone (Outlander, #7)

by Diana Gabaldon

One theory of how dowsing works is that the water or metal has a magnetic current, to which the dowser is sensitive. Your mother says that the first part of this is true and that, furthermore, there are large bands of geomagnetic force in the earth’s crust, which run in opposing directions all round the globe. Further, she tells me that these bands are detectible by objective measures but are not necessarily permanent; indeed, the earth undergoes occasional (every umpty-million years, I think; she didn’t know the exact frequency) reversals of its geomagnetic force—nobody knows why, but the usual suspect is sunspots—with the poles exchanging places. Another interesting bit of information is that homing pigeons (and quite possibly other sorts of birds) demonstrably do sense these geomagnetic lines, and use them to navigate by, though no one yet has figured out quite how they do it.