Oh, I forgot to mention the huge benefit of being able to burrow- there was a study with two different burrowing species and all controls were the same except one group had an earth substrate to burrow in. All the others developed bladder stones (this was juveniles) and the burrowing torts did not. Dehydration is controlled by burrowing. Chronic dehydration causes bladder stones. (I want to bring my guys to work and see if they have stones- I've always kept them so they could burrow, but they were kept in a lab for the first three years of their lives...)
Oh, but there's a vet in Arizona who is developing a new surgery to remove stones that involves an incision in front of the femur instead of having to cut the plastron open and bend it back- and the stone is crushed before removal, that's how they get it out through a small opening...
Nanci