How goes the excavations, Josh?
I've gotten sucked in down the hand-dug rabbit hole myself, slowly but surely, for similar reasons (or, arguably, lack thereof)! Mainly due to combination of good workable geology that's near a sweet spot of carveable X stable, desire for bulk fill material supply, and the appeal of year round passive climate control.
So far about 15' into an exploratory tunnel just to see how goes, and what's possible, but starting to dream bigger as the potential utility + feasibility expands. Not really thinking in terms of living space so much, but I suppose having a manageable 'bolt hole' in case of emergency (like fire) could be a real life saver. Originally thinking more for root cellar and general cool (non-freezing) storage. Currently most excited about the prospect of large volume underground water storage, since I can fairly easily channel a good amount of winter surface flow down in there, and the substrate is pretty near impervious to drainage as is. It's pretty slow going so I'm sure I'll be able to experiment and adjust over a few seasons to home in on good long term plan for it. Fun side project anyhow, especially during hot weather! Amazing to just climb in to 20-30F cooler temps like that.
Photo of current "R&D" status here: