Below ground/cave temp is generally the annual average for the region. I think up here that closer to 38 degrees.
Not sure I want to drill half way to the mantle just to get a decent heat pump going.
You only need to get below the frost line, and glom onto a chunk of thermal mass.
Then you can pump heat into or out of that same mass on an annual cycle. The extra heat of summer becomes the extracted heat of winter.
In your climate, I would supplement the heat storage by building a solar-gain window into the system. This could be like a cold-frame, but designed to absorb heat exclusively for storage underground. In theory, during certain parts of the year, you could get by with only circulating what you've got to where you want it. No compression needed.