Is this a "Dumping pit" by the builder and who is resposible to pay for this house was built 1998
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.)