The top was a vinyl linoleum, the second layer was 3/4 inch plywood, the third layer was a 2X4 frame and insulation, and the fourth is some kind of foam board. I pulled the foam board off one of the sections to make sure I get a record of how it was made so I can put it back together correctly.
The rats were living between the layers and had made them a home with the insulation.
This is all that was left of the insulation. This had to have been a huge rat.