Good guess. There's 2' of dirt on the roof with the rear wall totally buried. But beyond earth contact, I followed Passive Annual Heat Storage (PAHS) advice. The book was published in 1983.
The primary difference he came up with was putting an insulating umbrella over the entire structure that extends 20' beyond the perimeter. This keeps the mass (and inside air) temperature hovering near 70º all year. It also solves the major problem underground structures frequently had: leaks.
PAHS is a heating/cooling system, independent of architecture or building material though they both need attention to be certain annual heat storage will actually occur. Summers, the house dumps excess heat into the dirt mass, warming it while cooling the house. The mass is highest temperature just when the house starts to cool down in late fall. By early spring, the mass is coolest, just in time for summer cooling.
Takes a lot of mass, but it's stupid simple. No maintenance, no pumps or moving parts, no repairs, no energy to buy. We don't even bother with window coverings (no neighbors).
Cost, if the commercial materials I chose are used, is usually considerably below traditional stick-built. I built one for a client down the road. He needed a mortgage, the appraisal came in 50% higher than it cost to build: instant equity! Which was a direct comparison to stick-built here.
Which is why he's putting up with an unbelievable 80 mile each way commute. He says the job isn't forever, the house is. His previous house was a lovely post&beam SIPs place that he would never again settle for. Current owners think it's great.