Val, We had to go about 5 feet under the deck/steps and it wasn't a problem. I vote that you keep digging (especially since I'm not doing the digging).

You can hire a leak detection company to pinpoint the leak; that might save you some effort. They'll use sound detection equipment and seem to be generally well regarded. Look up American leak detection in your local phone book (they're a franchise type outfit and about the only game in town as I understand it).

Here's a picture of our tunnel: