Drainage pipes must be easy, I managed to find one and I wasn't even trying to dowse.
Whilst appreciating all the thoughts about the Earth's magnetic field, I'm sure that the answer to "How does it happen" is as much inside the skull as outside. I don't mean it's in the imagination, just that Science doesn't know half as much as it thinks it does about how the human central nervous system interacts with it's electro-magnetic environment.