I think you have settlement issues under your slab.
This looks just like a job we had 2 years ago on a house where the uncompacted soil underneath, sank 16 inches in some areas.
We discovered voids underneath after we picked some areas and drilled 3 inch holes into the slab. Sure enough, there where voids up to 8 inches underneath.
Only after we brought in the concrete saw cutters and removed the 3 foot pieces did we discover the larger voids.
The next step was to compact the existing soil before we poured the new concrete. The next problem came when we lost the compacter into a 4 foot deep hole in the center of the floor area, I mean , it just disappeared and we had to dig it out. Turns out, we dicovered lots of nails and debrie as we dug it out....looks like this was where they buried the trash for the job.
Get an expert out there, the shotgun effect as we call it, is only guess work at best. You could also have water issues under there that will never go away until you divert it or control it.
Good luck, and keep us in the loop.