I have a large rectangular area in my back yard. Over a year now, the county told me that by the pictures that I sent them, that is was a "dump pit" that the builder used while building the neighborhood. The county said that I should through all the seasons, let it settle through snow and rain and then fix it. Now, 11 years later I have a mess. My neighbor said that he can see wood and roots in there, I am too scared to get that close. Am I safe to just fill it with dirt? I live in TN and can't even get anyone out here to really tell me what it is. It doesn't appear to have gotten any larger, but it is quite long, probably the whole length of my back yard.
I hardly think it's Love Canal so I'm unclear why you are scared of it. Sounds like a hole in the ground with tree stumps, et al slowly rotting away.
Dump some dirt on and grade it; repeat as needed. Since it appears that you have left it for a long time and it's gotten bad, you will probably need machinery (bulldozer or track loader) to do the dirt the first time. I'm sure someone will be along to claim it's absolutely essential that you dig it all out, but so long as you are not building a house on top of it, I can't see a particularly good reason to spend that much money for a reliably smooth lawn. Just plan on getting a truckload of soil every spring for a while and fill in the holes.
If the "dump pit" was legal and aboveboard when done, you bought it, you now own it, I'd not expect any help from the builder to be forthcoming 17 years later; I guess you could ask them nicely if you can find them, and you might get lucky - but I doubt you can get anywhere useful in court - not that legal advice is really on topic, but I suspect you'd simply enrich some lawyers and come out with the same problem still unsolved and no hope of any help. Taking folks to court tends to cut off any help (that might be offered for the sake of decency, if you don't take them to court.)