My mud puppy or how do you stop a puppy from digging?

We have a 7 month old half golden retriever, half chow puppy we got from a rescue in northern Illinois back in November. His name is Marley, and no, we didnt name him from the movie, we named him because we like Bob Marley, and he looked shaggy in his picture on the website. He was an adorable puppy and is growing up into a great dog, except for one problem.
He has discovered digging. He saw Jack dig up some ferns to transplant, and figured out that if Jack could dig in the dirt, he could too. The back yard is starting to look like a bombfield.
We had even put a small child's pool in the backyard and put sand in it for him to dig in, but he just likes to lay in it. It was an idea we got from watching Victoria Stillwell, but this one didnt work.
Any ideas or suggestions would be most greatly appreciated... thanks!!





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