My idea..... take a Sawsall with a long blade and make a long cut in one direction..... turn on line and see where / if water comes up.
Two problems I see with that idea. If the lines were done "properly", they should be 10"-12" deep, so even a long blade might miss. But telecommunication lines (cable, telephone, etc) from the junction box to your house (or neighbor's house) is likely only 6" deep (if that). If one of those are in the way, you are sure to cut through it.