I guess I was probably in 5th grade.  I think the inspiration was a series of books I had about a lonely kid who invented stuff.  He went off and built a house in the woods and then the other disenfranchised pre-teens from his town came out one by one and he built them all houses around a little meadow in the woods.   My favorite though was the underground house.  Or maybe it just seemed more attainable than the other choices.  In any case, there was something about "being underground" that just appealed to me (maybe it's an instinctive thing harking back to the safety of a cave).   So I went out and dug a giant hole in the lawn under the apple tree right out by the road.   My parents didn't own the house on the Beaverkill, they rented half of it and the barn and pasture from a flyfishing club.  So they weren't too awful pleased when they noticed what I'd embarked on.  I think I'd made decent progress before they noticed, but they were kind enough not to tell me I couldn't keep digging (though I do remember a warning about damaging tree roots which probably prevented me from trying to expand the footprint any farther).  Anyhow, it worked out ok.  I believe I got a 3-4 foot square hole about 3-4 feet deep, and then I went and found a piece of plywood or something to cover it with and I'd sit inside on the cool dry dirt and it was pretty fun.   Not as fun as the drawing of the underground house in the book looked but still pretty good.   Of course the first heavy rain made it much less attractive, and I had one of my first lessons in entropy (it was quite amazing how much of that hole managed to get filled back in by itself -- by the time we'd left the Beaverkill, it had filled in to within about 1 foot of ground level.)