It's a mix of geothermal energy (extra heat), and insulation/thermal mass abitrage. Several hundred tons of rock/stone/soil are a great insulator, and are going

It's a mix of geothermal energy (extra heat), and insulation/thermal mass abitrage. Several hundred tons of rock/stone/soil are a great insulator, and are going to be consistently above freezing, which means you get a munch higher starting temperature for heatpumps if the alternative is freezing air.