The path of electrons in AC circuit?
While it's common practice on schematics for small (i.e., all in the same place) circuits to show grounds that don't appear to be connected, this is just a convenient way not to clutter up the diagram with all the return paths. 

If however you're talking about literally kilometers of physical ground, you'll find that in practice, at least with power delivery, there is always (afaik) a return conductor to ensure a low impedance to complete the circuit. In effect, this return conductor is in parallel with whatever conductance the actual earth connections afford, so some of the current will actually travel through the earth, but mostly it's going to be found in the wire. 

RF applications might get away without this return wire, but then RF applications can oft times get away without either wire, so I'll leave that aspect for someone else to tackle, should you be interested.