I think it's a great idea, I'd be hesitant to use too much wood underground. Have you thought about how you plan to enter the space from the outside as well as any reinforcement in the center to support the weight of the ground above it. I'm no expert but those are justtwo things that popped in my head
That's the kind of advice I am looking for. I plan to research for about 6 months and build it in the fall.