The idea that burrows would be more humid has been tossed around a lot, but it appears that at least some burrows are not noticeably more humid than the air- http://www.tortoisetrust.org/articles/pyramiding.html
The article now says: "Moderate humidity (50%ish or so) helps prevent dehydration but the substrate should be kept dry." I added the (50%ish) part to try to help clarify the goal.
The article now says: "Moderate humidity (50%ish or so) helps prevent dehydration but the substrate should be kept dry." I added the (50%ish) part to try to help clarify the goal.