Subject: locating buried wires and conduits by holding two metal rods that cross

A well driller showed me that he could locate buried objects through what he
called "witching".  He bent two welding rods into elongated "L" shapes and held
them loosely in his hands.  As he crossed known buried objects, the rods
crossed.  They uncrossed as he moved away.  I was skeptical, and tried it
myself.  IT DOES WORK!  I could feel the rods move closer together as I crossed
known items.  The looser my grip on the rods, the better they worked.
It worked on known underground wires, metal pipes, concrete pipes under an
asphalt road, pvc leach lines, pvc water lines.  The rods also crossed when near
overhead wires.
I asked two physics professors why this works.  One refuses to believe that it
works at all.  The other acknowledges that it works, but could not explain why.
My question:  What property of physics explains why these rods cross when
passing over buried objects?  Some sort of magnetics?

Re: locating buried wires and conduits by holding two metal rods that cross