What this is, is someone filled in an old in ground swimming pool with clay and didn't punch a hole in the bottom to let any surface or rain water escape, so over time the clay became extremely saturated. When I drove the mini excavator over it the clay began to "pump" or liquify and this is what happened. I have seen soil liquefaction before but never to this extreme. The homeowner was very concerned as you can imagine. I dug all the clay out and hauled it off, broke up the bottom of the pool and re-filled the pool with fill from onsite.