Alot of times dogs dig to get cool earth so the pool is a good idea.
What to do with magnificant dog hole. Fill with a couple inches of sand, cover in a thick gauge black plastic, cover in old river rock so a shallow pool forms that is easy to hose out and voila..the most awesome bird bath ever.
We had a shed that Mary, he sister Sally, and her Mom and Dad escavated under. THere was an entrance, and two exists. SAlly would grab one of my daughter's stuffies, shoot out the back door and race for all she was worth to the shed, zip under and shoot out the other side without hoping no one else saw her. So I would have to send my son who was about 7 out there to go down and get it before his sister had a hizzy. He came out saying "there's rooms in there!"
Mom Tate got stuck once in one of new exits and we had to get shovels to get her out. SHe was butt facing sky and stuck for all she was worth. SHe was embarrassed.
THe only way I know for sure to make sure they don't is to go out with them and stay with them until they learn not to.