This is what I was also thinking.
Just need to spec out a TON of lightweight flowable concrete. I also think I would approach the seller about having the line mapped to determine exactly where/how deep it is.
The owner had a survey company (who did a survey when he bought the home earlier this year) out there today to dig into it more. Basically, the current owner said he never would have purchased and rehabbed the property if he knew it was there. Sounds like title insurance, or the insurance for the survey people, might become involved. If it were filled in, wouldn't the street just flood in heavy rains? There is another inlet about 300 feet down the road. I wonder if they could trench over and tie into that. Not cheap, but I don't think any solution will be at this point.
