did you guys know that if you get 4 ft underground, it STAYS 50F degrees? Sure, you need solid shoring, but it saves you all of that firewood cutting and hauling, and is cool in the summer, too.
but you have to cut all that wood, every year, just to heat it. When if you go 5 ft underground, it STAYS 50F degrees, year round. No need of cooling or heating. Just some walls, insulation between them, and shoring. Save you 90% of the work making it and ALL of the work trying to heat or cool it. Wouldn't be hard to seal out the bugs, and bears wouldn't be tearing into it, along with no risk of wildfire. If you make it long and narrow, you wont need big timbers to make it, or to replace them every few years when they rot, either. Yes, you have to arrange for drainage and be careful where you locate it, bfd. A little side benefit is you'd not be at risk from fallout or bullets when you're in it. and you'd be safe from blasts, if they were distant enough. You'd also be a lot harder to find if hostiles are moving thru your area. Especially if you put a concealed lid at each end of your tunnel. wire a .22lr noisemaker to the lids, if such hostilities are the order of the day. A lot of the Viet Cong lived in their tunnels, at least during the daytime, for 30 years.
Doh! Just me again.