iPhone Compass Confusion

Taking the iPhone 3GS on the subway will repeatedly get its compass all out of whack. It sits and spins in all directions. Eventually it produces this screen:

Compass Interference

I don't know how the compass works in the phone. But I'd venture a guess that the big, giant electrified track might have something to do with the "Interference" referenced in the alert.

Fun stuff. Love the figure 8. Very classy.

Is that, like, a nautical thing?