This weekend, we broke ground to explore a settling corner beneath the pool at the rear of our lot. This year's heavy rains literally doubled the drop (previously 2 1/2 inches) of the pool deck towards the southwest corner to 5 inches.
This is on the downslope side of a 20,000 gallon concrete pool. The coping was discovered to leak, in an older pool without proper controls for water level, so that may be the source of the moisture underneath. (No other leaks known- no noticeable disappearance of water) Here's what we found:

Looking towards settling corner:

Below Remaining Pool Deck:

Hard to gauge scale in a photo, but that void is a good 4 ft wide & 5 ft deep!
Another View Below:


Since it's been there 50 years, we knew the pool deck was going to be a big project, but that kind of erosion, and the cavern it created, pretty much amazed us! The soil engineer has been added to our call list... then it'll be time to call in the Caterpillars!