Soil settles over time under concrete. Even properly compacted it will do that. I wish we had digital photography years ago when we cut coolant trenches into the floors out in the plant. Building was built in 1949 and 45-50 years later, there were voids under the floors in some spots you could crawl into. Realizing we had heavy equipment sitting on these floors, we decided to have a company come in that did a sonar check of the floor and then pumped grout into all the voids. The interesting side effect from all of this was the 5% drop in ambient noise in the plant.