How do you stop a dog from digging?

Our dog is digging big holes in the yard and I have no idea how to get her to stop. She is mainly an outside dog (by her choice) and I never see her digging. When I am outside she doesn’t do it because she is busy spending time with me. I’m sure some of it is boredom, but she has bones and toys – I can’t be out there all of the time!

Any suggestions?