> 4 months of winter buffered by 10 ft of ground!

I'm sorry, but you write this as if that's nothing. Making a 10 foot hole is a massive amount of energy being spent. It's a massive amount of weight as 1 cubic yard of dirt is roughly one ton. In 10 cubic feet, that's roughly 3.5 tons. I say this as someone that moved 6 cubic feet of dirt by myself with a shovel and a wheelbarrow.

So to think of 10 feet of dirt as a slow insulator would have to be one of the worst insulators out there.