This is what we found.
That's right, an 18x24", 4 inch thick slab of concrete about 2 inches below the surface. I guess that is why the grass would not grow! Jeremy is outside re-sodding the yard right now. We also found a t-shirt, an orange plastic net, ketchup packets, a McDonald's cup and a few chunks of metal. I am wondering now if we should dig up the entire yard and go on a treasure hunt!