Thanks. No, I don't have much of an underground guard. The pen is buried about 3" into the dirt but that's about it. I've been keep these same turtles for over 20 years in the same type of pen and I've never had one try to dig out. The trick I found to keeping turtles from escaping is to trick them into thinking they're not penned in. When a turtle come to an obstacle it's either going to a) go around it, b) dig under it, or c) climb over it. To keep my turtles from feeling like they're not penned in I use large spaced rabbit fencing that is green in color.
Here's an extreme closeup of the wire I use.
I used pressure treated lumber for this pen. The new stuff doesn't have all the bad copper/arsenic in it like it did in the past. But if I was to build a new pen I would use that new synthetic wood that they use for decks that's made from recycled plastic. Obviously this stuff with last for pretty much ever. It's a little more expensive but would be well worth it.