Collapsed basement slab - Advice?

[Bob C.]

Well... the things you see in this business. This was a real eye-opener for me. The house has been on the market for about a month. From the pictures, it seemed priced well and was wondering why it hasn't got a deal yet. Had to visit to find out "the rest of the story". Walking around, all looks good. No foundation cracks, everything in good shape. Enter the house and head to the basement... AH HA! Mystery solved.

The basement slab has collapsed. The center has sunk about 18-20". I felt like I was in a poltergeist house and the house was getting sucked into the earth. I'm in the Northeast and we don't have sinkholes.

Foundation all looks good. No unusual cracks.

Center supporting foundation looks good.

1st floor is level, no cracks in the ceiling or walls.

It's a 1974 raised ranch

There's no pipes or electric in the slab

Looks like this happened 10-20 years ago

My Theory:

The lot was obviously built up, bulldozed, filled. I'm told that, in the day, sometimes the tree stumps from clearing the lot got burried under the slab. I'm guessing that either the ground was not compacted or there was debris that decomposed and left the slab floating. Eventually, the slab breaks and falls into the hole. The foundation is Ok because it's deeper, presumably on "real" ground. Heh... just had another thought... that if the contractor was this cheap to short-cut fill, the slab is probably really thin too, assisting in the breakage. (Hmmn... and the more I think about it, I didn't see any steel in the slab)

If this is the case, it's "simply" jackhammering the slab, removing debris, adding proper fill (maybe having to remove other debris that's still there) and repour the slab. Fortunately there bobcat is access to the space via the garage.

How does my theory sound?

Anything I should be thinking about?

Run away? Or do you smell a deal?