I started thinking about this when looking at how metal filings align themselves with the magnetic field of a bar magnet. Since the earth is comprised of a great deal of iron, it seems to make sense that the iron in the crust would attempt to align with the geo-magnetic field. There's iron in seawater, the earths core, and even our bodies for that matter.

The conductors of ULF aid the telluric currents Lowell mentioned.