Maybe something underground is full of water or ants or something. Possibly the wire insulation got nicked.
There are some fun instrumentation possibilities, like probing the soil at the surface and measuring/listening for AC potentials.
A map of the surface potentials of some chunk of land, color coded by frequency, would be fun.
We were on the roof yesterday and speculated how cool it would be if we could see RF, all the microwaves and cell phones and wifis and transmitters. That's not possible. But a surface potential map is.