Yesterday I came home to find Apache had buried himself into the substrate. I use cypress mulch, and you could only see little patches of Apache skin in between it; he was fully submerged in substrate.
I am glad that I do not keep my heatpads too hot; I measure the temp of the tank floor glass, and just put a little layer of substrate over that part. Otherwise this burrowing brat could have roasted himself. I have never seen a boa do this, have you?