here are some of the reponses:
Tammy at The Crabbage Patch has been answering questions about her outdoor crabitats. She says that the large PPs will dig down to the bottom of her enclosures, which are 3 feet deep, and she thinks they would go deeper if they could. And the ground that far deep is really cool, maybe 60 degrees. I think I read somewhere that the hermies prefer to moult in cool substrate. I've had better success with heat lamps than I ever did with UTHs.
Yup, I emailed Tammy, here's that thread:
Digging HabitsI didn't think to ask about soil temp, Frowner she lives in southern Florida, so I guess it's not much of an issue there.
It would be good to check soil temps from time to time just to see if it makes that much of a difference, and how it varies from the air temp.
If anything, the UTH could counteract the effects of the damp substrate if placed on the bottom and the crabs were molting near it.
Crabs need humidity and darkness to molt properly. A warm environment is essential to their health, yes, but crabs wouldn't dig to an area where they would feel it is unsafe to molt. They are looking for quiet, moisture and lack of light for a month or more, all which are much harder to find at surface level.
The biggest issue is that the UTH dries out the substrate around the UTH, making that area unsuitable for molting. It also bricks sand, which could be dangerous if you have the UTH on a timer or rheostat.
Plus relative humidity rises as temperature decreases, so the crabs will have higher relative humidity digging down into cooler, damp substrate, which should help their molting process.
100's of happy molts in cool, damp substrate can't be wrong!
