Digging to China

So, all of the litterboxes around here are HUGE. I found an awesome one though. It looks like an igloo! I set it all up, and found a heavy thin box that works well as a step, and used extra litter in there cause Dozer's such a little guy right now. He figured it out and everything right away. The only thing is that now when he uses it, he DIGS and digs and digs and digs. Only one hole, but as deep as he can. Then he goes in the hole and proceeds to COVER and cover and cover and cover. Then he likes to stay in there and play. Is he digging to china to keep his play place free? Is it okay for him to play in there?

Oh, and by play, I don't mean that he's doing anything harmful like eating the litter, or messing with his poop. He just likes to jump around in there, or paw at the walls, or just sit and relax.

My Rosie digs to China as well. The deeper the litter the deeper she digs and when she digs she manages to sweep it out the box as well She doesn't really cover her poop up though so Sophie does it for her.

When Sophie digs she's a little lady and does small digs, but when she covers it up she covers it that much she's made a hill in the middle of the litterbox

Sophie used to play in her litterbox but grew out of it.

My baby Francesca, a senior cat from the SPCA was having difficulties with the regular box and litter that came with her. Yup, she was digging, to China. I did much research online, and with this site, and with the SPCA.
I ended up with a large tote,she is very large, and walnut litter.
So now we were still headed to China, but because there was a cover on box, with large hole for easy in/out, she holed up in there!Granted she was new...but I finally took the top off.
She is still digging like crazy, but has gotten more social. So sensitive my baby Francesca.She loves to borrow under covers,as photo shows.
Any thoughts, or let it ride (to China).