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Dug into the ground, we usually call a "ground source heat pump", or less accurately, "geothermal". The normal split systems are "air source heat pumps". AC is a heat pump without a reversing valve.

A heat pump is not necessarily dug into the earth. Rather, the flow of the heat pump is moving heat (thermal energy) from outdoors to indoors or the other way around in an air conditioner.

Depending on the direction of the coolant flow, you get either a indoor heating or cooling unit. This is best demonstrated by going in front of the outdoor unit of a heat pump, when they are cooling, the outdoor unit generates heat because it's compressing gas, which then is then expanded when it reaches the indoor unit, generating cold. Exactly like a refridgerator.

There's also air-to-water retrofits for houses where you had centralised gas/wood heaters and water radiators.