Thanks for the replies all. My guess is now that he slipped through one of corners where it is a little flexible. Sigh, I thought that after 3 months it would have been safe but I guess not. I made straps for my ball pythons enclosure (you can sort of see them in the above pic) because I knew they were pretty strong. I never thought that tiny corn would be able to push up like that. Oh well, live and learn.